I'm using Windows 7 right now. Boot times are faster, FPS in games is slightly lower.
There are so many little niggles that I have with XP that are fixed in Windows 7. For example, you can now move maximised windows to another monitor without an addon. Or when watching videos in full screen in Windows Media Player on your primary monitor, the taskbar no longer floats on top if you alt-tab out. Oh, and I think libraries are godlike.
Your PC will probably run Windows 7 just fine, but I think the CPU/GPU will be knocking the low 2-3 scores...
Not worth it just now anyway. RC1 is still a bit buggy. Sometimes when I lock the screen WoW will just exit. And it appears it won't resume from standby properly, either that, or my ACPI tables on my new motherboard are broken.