You can go wrong if he spends that much money. If he's going to spend that much on a CPU, he should get a current gen model. Lets say both the 9550 and the 2500 are $200. He should buy the 2500, sell his current setup for $100 and use that money to get the motherboard. He then spends $50 on 2x4GBs of ram and he's set. Spending $200+ on a 3 generation old CPU is dumb.
Getting more ram isn't a bad idea, but I wouldn't do it. First, you have a CPU issue. Second, you're probably running DDR2, while DDR3 is the current standard. Its a bad idea to buy old stuff, see above rant.