I went from a 3200+ to a 4200+ X2... nice increase. But buy what you can afford. The 4400+ is very nice with double the cache, just good luck finding one. I was on Amazon earlier and they still have a lot of X2 s939 listed.
Since I just skimmed the posts above I might have missed it, but going with 2 gig of RAM in dual channel is the way to go for speed. 1.5 unless it is 2 x 512 and 2 x 256 ain't running dual channel... shoot for 2 x 1 gig.
Another thing that will help your speed is some serious defragging. And I don't mean the defrag that comes in Windows. Grab O&O or Diskkeeper and do some real defragging.
Make sure your system is clean of any malware, clean up the registry, and don't have anything running in the background that doesn't need to be running.
Go with these tips and you can make your current system last for a while yet.