|You might be able to get away with the 300W PSU but if it's a Bestec, you'd be better off replacing it with a 400W Corsair.|
If you're looking for a gaming video card, check this article:
Best Graphics Cards For The Money: July 2010
"2GB RAM - Will upgrade to 4GB in the future"
You *probably* have 2 x 1GB now, so to upgrade, you'll have to pull the RAM you have & replace it with 2 x 2GB (preferably DDR2-800)...that will cost you about $75. But if you're going to stick with Vista 32-bit, there's not much point in getting 4GB.