Sometimes, after awhile with a lot of installing and uninstalling, you get stale files left all over the place or parts of the applications get corrupted.
I usually do a once a year reinstall and that seems to clear things up.
Have you also done a memory test to see if you have bad memory? You might also want to look at your swapfile (if you have one). Sometimes after awhile, these can get really fragged and that can cause some issues, especially if you're swapping out a lot. Although, with 3 gigs, you shouldn't be doing bad at all, but it's still worth a look.
When you notice the choppiness of the games, do you see if your hard drive light is flashing a lot? If so, this could be a sign that you are swapping out to the hard drive.
Have you installed any new software that might have installed services? I would check to see what services you have installed and are running.
You could also try AMD's Fusion utility. You can set up different settings for it to use based on what performance you want. What you do is run Fusion and choose what performance you want and it goes and starts shutting down all sorts of services. Once it's done, launch your game and see if you see a noticeable improvement. If so, then it's definitely because you have something running and eating up processor cycles and/or memory.
Once you're done playing your game, be sure to stop Fusion so it will restart all your services again.
You can get Fusion at http://game.amd.com/us-en/drivers_fusion.aspx?p=1
If this doesn't seem to improve the performance, I would look at backing up your user profile files, game settings/saves and do a clean reinstall of the system.
Another thing to look at is, do you have system restore turned on? With all the installing/uninstalling, you might have a lot of restore points. I would get rid of them or else, go back to a restore point where you WERE running well, taking note of what you're rolling back to try and determine what might be causing the problem.