I was considering buying a new pc for myself, but decided to wait for about 6 - 9 more months. Just a couple of reasons why.
1. Windows Vista will come pre-installed, save yourself some $$
2. Windows Vista is 64-bit compatible, so make sure you get a 64-bit processor. Even though a lot of apps don't run at 64-bit and thus don't really benefit from it, it is still good if your OS is running as fast as possible.
3. Windows Vista supports and adds security to SQL Server 2005