There is no reason in the world why that machine isn't flying! Did you use a distro to install? Vanilla is the best way to go. And yes, Lion is rock solid for me...so is Mountain Lion but then I'm picky about my hardware.
I believe I used this guide to install Mac on my PC. Also note that I do have my mac install on a separate hard drive (128GB) and not my primary (Windows 7 installed (500GB)). Note I am running Lion. Yet still, somehow, my MBR was missing. The actual BOOTSECT (or bootmgr) was missing from my Windows hdd. I use Andy's custom boot method for multibooting. I do not know how to change that to Chameleon or any other boot manager, although I believe Andy's boot manager works out quite nice. Additionally I do not recall the approximate Snow Leopard .dmg I installed, but I am 99% certain it was retail. Completely vanilla.
On a desktop PC, highly recommend installing OSX and Win on separate hard drives which eliminates the possibility of having any mbr problems or boot loader tug-of-war.
My graphics card does perform excellent on every game; on Windows. The mouse delay I am referring to is the notorious hard-coded 32ms mouse lag (as seen here - mouse annoyances). I still experience this issue.
The graphics card you have should rip through any game on the market if set up properly. There is no mouse issues layered in either my Lion or Mountain Lion installs, and I've peeled back all the layers Only similar issue would be possible bluetooth mouse lag during heavy wifi traffic which can depend on many variables such as router location.
I haven't ran my mac partition in a while, but I do remember that after leaving it on for an extensive period, say 2+ days, it freezes up. That's a huge problem for me because I tend to never shut down or restart my machine unless absolutely necessary.