I don't know what his current HDD is as it's not listed. I just assume it's a 7200 drive. It won't necessarily help him in gaming (except for loading maps quicker - If game is on SSD) but it will really help in bootup and general user experience. There aren't too many options, but if the money is burning a hole in his pocket, a 30GB would be perfect.
I don't mean the largest possible, I mean the largest boost. GTS250 to GTX series card under 300 isn't a terrible large boost. Upgrading to an E8400 would be a fair but not very significant boost. 2GB more RAM, same thing. A new motherboard, sure - but would pretty much dictate a new platform, or a better CPU. By then, he has spent the 300 dollars.
I justify my statement in this:
I'm not satisfied with the overall performance though.
SSD is an "overall performance" (not really in games; Unless on drive) upgrade.