My comp is built for XP.
I haven't got the RAM for 7.
Nor do I have the vid, proc, or anything else for that matter.
I work my buns off for non computer type stuff. Fancy rigs are out of my scope of priorities.
Eventually I'll be up to MWO standards, but I'm not pressing for it.

Talk to some of the Guru's over in the Hardware section of the forum. Those requirements aren't that extensive. The CPU, GPU, Motherboard, and OS to run that should only be around 400$ Total if you put it together yourself and you have all the periferals. (Speakers, case, keyboard, mouse, monitor, etc.)
You should be able to save your HD and cd drive and possibly your ram, but 4 Gb of Ram is only around 50$.
If you keep your hard drive you can get the Windows 7 upgrade license for half the cost of a new OS. (99$)
You'd be surprised how good a computer you can build for not all that much money. (Especially compared to buying one from the big sellers.)