Don't know if you need an OS or not, but if you do, you can pick on of your choice. I would probably go with whatever you can get your hands on cheap for now, then switch to windows 7 when it comes out. Total of this system is $1768, so it leaves you well within your budget. You could add another HD for more space if you like, or add another one and run RAID 1 for data security. There is some room to play...
If you wanted to spend the money, you could upgrade to the i7 940. I don't know if it's worth the extra, but you would still be close to budget. If you have any sensitive data, you should probably buy the extra HD and run a RAID array that provides a mirror or parity backup. This will protect against a HD failure and complete data loss. One other suggestion would be, if you don't need a lot of HD space, buy the 300Gb velociraptors instead of the cavair black. It is more expensive for less storage, but they are faster. It all depends on what you want to get out of it.
Going of topic a bit here but i have been looking for pc parts for a number of months, i just noticed that that rig is very decent and for a good price. I was looking at a similar build but i live in Australia and the prices are much higher.
If i remove one of the 285 nad replace the ram with this: