You'll need the following to run Windows 7
1ghz processor (Either x86 or x64)
1GB of ram (2GB 64-BIT)
DirectX 9 Compatible Video Card
16GB HD Space
Keep in mind, that's the bare minimum. If you want to have access to all features you may need more resources... I.E.
Aero and GFX Intensive APPS >> Decent GFX Card (256MB Ram, DX9, PCIe at the minimum)
XP Mode >> Extra 1GB Ram, Extra 15GB HD Space, Processor that supports hardware virtulization w/ Intel VTor AMD-V turned on.
In other words, according to MS, you're not capable of running Windows 7 at the moment. You'd need an extra 512MB of ram and possibly a new video card and/or hard drive. If you have the cash, Id suggest building yourself a new computer. You can build a rig that'll play almost any game out there for about $500. (That's not including monitor, keyboard, mouse,speakers, headphones, etc)