Computer Crashing, Refusing to Boot

After play 3d intense games like counterstrike for a while (about an hour) my computer is suddenly freezing, and emitting a high pitched buzz through the speakers. When I go to restart it by turning it off and turning it back on, it wont boot at all and emittes a computer beep half second long repeated over and over again. The only way for me to fix it is by resetting my bios, and disconnecting the power supply.

Here's what I know: This started happening since I replaced my current stick of kingston valueram PC3200 512mb, with a kingston valueram PC2100 256mb for testing for a friend. When I put the PC3200 RAM back in, this started happening. Mt chaintech VNF3-250 motherboard SHOULD support both types of Ram, however.

Secondly, the computer beeps I hear when it wont boot are the post error code (IBM) for memory failure. However I have a pheonix bios on my computer (Wierd).

I also know that the PC3200 recieved MANY errors in Test 7 of memtest 86+. However it had these errors before this started happening.

I just want to make sure this is DEFINITELY my RAM causing this, because I plan on buying a new stick and RMAing this one.