You would be able to transfer your number to a postpaid (monthly) account. However, your phone book will have to be manually typed in. You can get a prepaid account with no contracts, but you won't be able to keep your number.
On a postpaid plan, the Sidekick comes with the Sidekick Unlimited data add on package, which for $20/month on top of a voice plan, it gives you unlimited e-mail, web browsing, IM, and text messaging. You can also get the Sidekick without a voice plan, where you pay $30/month for unlimited e-mail, web browsing, IM, and text messaging, and calls are $0.20/minute.
I would suggest purchasing a Sidekick 3 instead as it has an immproved camera, a music player, a slimmer form factor, and MUCH faster data. If you do, however, choose to buy a Sidekick II (which is no longer sold by T-Mobile), you should call T-Mobile or visit a T-Mobile retail store to receive your
SIM card (it's this card behind the battery T-Mobile and Cingular phones use) and transfer your number, as well as pick your plan.
Good luck, and have fun with your Sidekick!