Installing windows 7 on my new system was simple enough, but expect a load of updates after you connect to the internet. I built a new system a couple months ago because I picked up a copy of Starcraft 2, then found out my 6 year old Dell wasn't ever going to be able to run it.
I got a 3 core AMD Athon II processor and a GE Force GTS 250 video card. New toy has 4 GB RAM plus another 512 MB on the video card, with open slots on the motherboard to hold up to 4 more GB. With what it has in it now it runs SC2 with all the settings maxed. I figure with some more memory added eventually it should be up to speed for a few more years anyway, then I'll upgrade the processor if necessary. I put in 2 612GB hard drives and 2 DVD/RW drives. Only hangup aside from a DOA processor that had to be returned was that my ASUS M4A785-M motherboard doesn't have a connector for a wired mouse, so I picked up a Logitech wireless optical mouse. Then a few weeks later I snagged a used microsoft wireless keyboard/mouse on e-bay dirt cheap so I don't have to swap the keyboard back-and-forth. Now I just need a new, bigger flatscreen moniitor...
I hope you bought a good power supply, a guy at the lfs last week was crying about a cheap one melting down his new system.