The Q8600 would be about as good as most AMD quad core processors in the 900s.
By that I mean people really don't like them that much for gaming. Everyone always suggests to get the newer generation Intels which do a whole lot better per core than the AMD Phenom 2s do.
Something like a G630 would perform a whole lot slower than the Q8600 in anything that can use 4 cores, but most games only use 2 so it would come out ahead of the Q8600 in gaming by a lot.
There is no good reason you can't game on the Q8600, though. At least you would have the advantage that the processor would have unused cores which would potentially reduce some of the bottleneck from having a processor that old.
I generally wouldn't suggest pairing a 79xx card with a LGA 775 motherboard, but it should technically work.
Anyway, if it were me I would probably just try to ebay the Q8600 and move on. It is a good processor and it will work fine, but I am not a big fan of supporting technology that old. Doing so involves bringing forward a lot of baggage sometimes and even a great processor that is really old will be held back by all the other really old support parts you need to go with it.
I would say it is probably more trouble than it is worth.
A standard i3-2120 would blow away the performance of the Q8600 in everything and you wouldn't have to use old parts with it. The i5-2400 would make the Q8600 cry (though not much more than the 2120 would in games).
If your budget is really low and you really want to try to find some way to support the Q8600, I can try to work with that, but if your budget is any kind of reasonable I would re-iterate the ebaying idea.