Not sure how many your trying to box or want to, but upgrading the ram wouldn't hurt if your running out now it would be paging to disk and be rather slow. That should be fine for 2 boxing, 3 boxing would work but be a bit chunky...
What ever you upgrade your not going to get significant performance without upgrading RAM,CPU, Motherboard and potentially Power supply... As Fenril said, replacing the lot would be the best idea going forward though.

Not sure if you can get/put a 4x core amd cpu in that mb.