A lot of good info here:
Xbox 360 Console Resource Center : Game System, Specs, Accessories, Price, Games List on GameSpot
I made the switch from Sony to the Xbox and I don't regret it one bit. If you like online gaming the 360 is the best way to go right now, Xbox Live is so well done it's not even funny.
The premium console is probably your best bet and it sells for $349 and includes a 20 GB hard drive, wireless controller, and headset.
There's a table on this page that shows the difference between all the different versions.
Xbox 360 - Wikipedia, the free encyclopedia