The thing is, you wont be able to play all games, escpecially the latest ones on very high quality settings.
1GB RAM is the minimum you should have for Vista, but i would reccommend 2GB RAM.
I would reccomend upgrading the motherboard (if it doesn't have PCI-Express), CPU and RAM with a decent PCI-Express graphics card, that is if it is in your budget to make a decent gaming PC.