You mentioned you've already got 4GB of RAM, and a dual core CPU, so as long as you aren't expecting to max out Metro 2033
... my suggestion is to go with Win7 64-bit, and a new GPU that's at least a 5770, or even a 5750 would be a more sensible route for 1080p gaming at low to medium settings.
You can probably sell your 5670 on Kijiji or Craig's List to help make up the difference (that's what I always do, makes GPU upgrades much easier
Check the power requirements for the 5770 and be sure your ~450w PSU can handle it first. And if it's not a decent quality PSU, I can already tell you, it can't.