This is the very reason why I recently upgraded my music workstation. I got plain fed up with freezing and unfreezing tracks all the time, and I eventually lost interest for a bit. I probably should have upgraded sooner, but I also ran out of "inspiration" at the time, and I didn't produce anything of note (no pun intended) for over four years.

Can I ask a personal question? How long have you had your particular set-up? It may be that, as the software has developed, and your collection of soft synths and stuff has increased, you need to take the plunge and get yourself some new hardware... Easier said than done, with ever tightening budgets. But at the same time, there are some great PC deals out there to offset any recession woes. 

So if I were to increase my RAM to say, 8GB, what would be the minimum (or least powerful) mobo/cpu model that could handle it?