while the 550ti would be bottlenecked by the cpu, it would still make a huge difference in gaming performance. Instead of wasting money on a gt240, then getting a new system, it would be better if you get the 550ti, then get a new system to put it in.
I dont think he has that much $$$ to spend by the sound of it, getting the 550 would be a waste. If he wants a cheap upgrade, just a gt240 is fine, he wont notice a difference with a 550ti (crap card by the way a 5770 or 6770 is better and cheaper and uses less power) as he is bottlenecked by his slow CPU. Even a gt240 would be bottlenecked a bit by the cpu.