My Fusion experience
First - I am not a gamer.
I have a mac pro with 6GB of memory. I loaded Fusion and created 3 VMs for WinXP, have allocated 2 processors and 2GB of memory to one, 1 processor and 1GB to the others.
I have not found a single problem with Fusion. It is very fast, runs everything I've thrown at it, and am happy with it.
Why 3 VMs? Well, I replaced 2 windows machines with the mac pro. I set up a VM for each, used MoveMe and replicated each physical machine in a VM. When I want to do something that was on machine 1, I fire up that VM. Same for the other. Both can be running at the same time, while I am doing other work in OS X. Resources can be assigned and changed on the fly, moving a CD or USB device from machine to machine.
When one gives a bluescreen or asks for a reboot, that VM does it while the rest of the machine keeps right on running.
The third XP VM is for experiments, beta code, sketchy downloads/websites, etc. If it has a problem, it is not a big deal to go back to the clean snapshot and recover immediately.
Fusion has made my transition to mac painless.
(I never tried Parallels, so I can not compare them, I tried Fusion and it met my needs so I stopped looking.)
Fred