Now, that is a catch-all sort of heading, for what is good for one may be total anathema for another. If you are an avid gamer, you need a high performance PC, while a person who uses a PC for checking email and using the word processor can make do with a lot less. The configuration I have listed below is for the average family, where a little bit of gaming goes hand in hand with writing a few letters and maybe burning a couple of CDs of holiday photos. Of course, you can use this as a framework, depending on your budget, and play around as much as you want. The reason I wrote this is that I found that the cost of an assembled system goes up considerably if the vendor figures out that he is talking to a green horn.
Processor
If you are a gamer, you may want the latest and the best, which usually costs a packet, but most folks should be happy with an Intel Pentium IV 2.6 or 2.8 GHz. They give almost the same performance as a 3GHz processor, but come much cheaper.
Motherboard
There is a plethora of motherboard manufacturers, who deluge the market with an array of motherboards ranging from the cheapest to the most expensive. The latter are again meant for those who spend hours playing the latest games. If you want an average board, pick an Intel original board. An 865 or 875 series board should serve you well, unless you want it to support the new PCI Express format, in which case you will have to shell out considerably more for a 9 series(915, 925 etc). The presence of good on-board graphics also allows you to skip a graphics card, if you are not too particular about the graphics quality.
Things to look for:-
On board sound(preferably 5.1 channel, so that you dont have to buy a sound card), graphics and 10/100 ethernet(essential for cable modems)
Adequate number of USB slots for those printers, scanners, digital cameras and what not. Check that the board supports USB slots on the front of the cabinet(tower) so that you dont have to go wandering around the rear of the computer every time you want to connect a digital camera. Minimum 4 USB slots, 2 of which are in front.
Adequate number of PCI slots for any expansions you want, such as an internal modem, TV tuner card and so forth. I would suggest a minimum of 3 slots.
At least 2 slots for RAM modules.
An AGP slot is essential if you later decide to upgrade to a graphics card. Please note that the newer boards which support PCI Express will not have an AGP slot as the graphic cards are meant to go into the PCI-E slot(though there are not many graphics cards in the market right now that support PCI-E). Make sure that the board supports at least AGP 8X
Hard Disk Drive
Buy the largest, and the cheapest that you can get, which has a 5 year warranty. Also ensure that the disk runs at 7200 rpm as against 5200 which was the standard for older HDDs. Go for at least 80GB as the bare minimum, if you plan to store some songs or films on the disk. It costs only a couple of hundred more than the 40GB one. Remember that SATA is the way of the future and buy a SATA drive if you want it to be future proof. Seagate, Maxtor and Samsung all have their supporters.
Memory
The criteria here is to buy as much as you can afford. Most operating systems(Fedora core 3 or XP Professional) need at least 256 MB to run smoothly, but really zip along if you can put in 512MB. Go for more, if you use graphics-heavy software or games a lot or are into multi-tasking. Go for DDR 400 if you can afford it, stick to DDR 333 if you cannot. Kingston is the king of the lot, but Hyundai will do for the masses;-)
Cabinet or Tower
The watch-word here is the SMPS, which supplies power to the computer. They may support 250, 300 or 400 watts, and you will probably need one of the latter two if you want to future proof your system. Make sure the tower also offers an additional slot for a cooling fan if you need to add a second fan in the future(which may happen if the computer resides in a hot environment). Also ensure that the cabinet allows you to have USB ports in the front.
Floppy disk drive
I personally havent used a floppy in over a year, but all cabinets come with a slot for a floppy drive, and you may need one to rescue you from a system crash if you dont have a bootable CD. Since it costs only about Rs.500, you can put one in. Any brand.
CD/ DVD Drive
To keep costs down, I would suggest a CD-writer cum DVD reader combo-drive for most home users. There is not much to choose from between the various brands available in the market, though LG and Samsung seem to be very popular. If you do a lot of CD writing, it may be better to buy two discrete drives- a CD writer and a DVD ROM. If you plan to store gigabits of data, you may want to invest in a DVD writer, but check up on the nomenclature(+R -R and so on) before you put your cash down.
Monitor
Get a flat screen, 17 inch monitor if you can afford it. of course, 19 inches would be better if you plan to watch movies on it, or plan on buying a TV tuner card. TFT monitors are great for looks and for saving space, but have a limited viewing angle. Plus, they cost a bundle.
Keyboard and mouse
Opt for a good multimedia keyboard, which allows you to start your music player, open the My Computer folder etc using special keys. I-Key is a good option for those on a budget. If you can afford the best, go for the cordless combos from Microsoft or Logitech. In fact, their lower versions are also good buys, but will set you back by about Rs. 2000. As far as possible buy an optical mouse-it works better than the ordinary ones, and do not accumulate dirt. Remember that you dont have to use a mouse pad with an optical mouse. In fact, your cursor may misbehave because the mouse misinterprets signals that bounce off colorful mouse pads. If you prefer the ordinary mouse, try out I-ball, Microsoft or Logitech.
Extras
Graphics cards, sound cards etc are optionals, according to your budget. If you have invested in a DVD ROM, and plan to watch movies on your computer, invest in a good set of speakers(at least 2.1 channels). If you pride yourself in being a good bargainer, talk your vendor into giving you home printer and UPS(absolutely essential in our land of eternal power-cuts) for free.
The shopping list
It should look something like this
Intel P IV 2.8 (or 2.6)GHz processor
Intel original 865(or 875) motherboard
Samsung 80GB 7200 rpm hard disk drive(or 120GB SATA)
256MB of DDR 400 RAM
Logitech(or I-Key or Microsoft) multimedia keyboard
Logitech(or Microsoft) optical mouse
52X CD writer+ DVD ROM Combo-Drive
Floppy disc drive(Sony/LG)
Tower/ Cabinet(Intex, Antec)
Creative Inspire 2.1 speakers
LG Flatron 17 inch monitor
UPS from APC.
This setup will cost you about Rs. 25-30, 000 depending on your geographical location and bargaining prowess.
Good luck with your new purchase!
If you feel I have left out some detail, post a comment and I will try to rectify it.