And it's only these specific games that you have (crashing/freezing) problems with? Have you tried out a hardware-heavy app like Doom 3 (or the demo), 3DMark 05, or DX9 RTHDRIBL 1.2? If not I suggest you do, as I think heat is definately an issue here if your comp freezes when under stress.
Otherwise, you should realise that 512MB of system RAM is kinda low. You'll be using a hell of a lot of virtual memory (on your HDD). I'd recommend no less than 1GB of good quality RAM for gamers. Check Task Manager (Ctrl+Alt+Delete) while running intensive apps to see how much memory (and CPU power) you are using.
AMD X2 4400+ Toledo 1MB/Asus A8N-SLI Deluxe/2GB Kingston HyperX/SLI Gigabyte 7800 GTX 256MB/Creative SB Audigy 2 ZS Plat w/ Creative Inspire 6.1 speakers /180GB Hitachi PATA/19\" AG Neovo F-419 LCD/OCZ PowerStream 600W PSU/WinXP Home SP2