the Quadro 120M has 256MB of Dedicated memory and can dynamically share an additional 256MB with the system. That being said I do beleive the X600 is a slightly better performer than the 120M Quadro....that is until I can find a way to hack my D820 to a Precision M65 BIOS. The Quadro 120M and the Quadro FX 350M are the same hardware. Just a different BIOS.
I'm going to search the Internet for benchmarks. I ordered three of them: one for me, one for a colleague, and one for my boss. I hope my boss isn't disappointed. He and I both have D810s with X600 right now.