On these systems you can probably only do CPU mining... without much success. As for upgrading, you'll probably be interested in getting the best GPU / U density. If you can afford it, Supermicro has nice systems that can host 2 double-size/full-length gfx cards in 1U: http://www.supermicro.com/gpu/
However, do the math. The hardware is going to be *really* expensive and it's going to be a reaaaaaaaaaaaaaaaaaaaaly long time until you will recover the invested money (if ever). I was in the same situation as you and I dropped the idea in favor of CPU mining (otoh, I don't have old P4s).