The performance difference will be large, so I would say that since you have the processor, you could get the MOBO and RAM for fairly little.

DDR RAM is up right now, but it has come down from it's high of around $80, I've seen it going now for around $50, but it fluctuates a lot. It's almost like the price of pork bellies on the commodity market.

I would say wait until DDR RAM goes down a little bit, you could save $30 or $40 dollars if you're buying another 512MB, but that is really a pittance for some.

I would say that the best bang for your buck is there. Go for it.