Actually using system RAM as video RAM is slower than a card having it's own RAM.
A video card with 512MB of it's own RAM will be faster than a card with 256MB that's using 768MB of system RAM. It's all about the latencies there, and it's simply a far shorter distance from the card's CPU to it's own built-in RAM modules than it is through the system bus and to the RAM on the motherboard.
Anyways in FRAPS choose to record uncompressed (full frames if that's what it calls it) and see if that hurts your FPS as bad.