I planned a trip to gangtok in Feb 07 and got every thing booked through https://sikkimtourism.com/index.php. The travel agent has a very impressive website. Sikkim is a beautiful place but be careful during winters; as sudden weather changes are bound to affect train / flight schedules leading to delays & even cancellations.
The weather is also very cold & guys visiting the place must take heavy wollens especially in winters. also be aware that some places in & around gangtok & sikkim cannot be visited without RAP (Restricted area permit) and also on certain days of the week. Also guys certain parts of the city can only be visited by a local jeep / taxi permitted by the defence folks. The Northern part is damm beautiful but not quite motorable in winter. also heli tourism is also available in gangtok & one can enjoy several rides if u cab afford the same !