I have an older system (about 5 years old now), running an LGA775 motherboard with an Intel Core2Duo 3.0ghz (OC'd to 3.6). The system is very stable, so no worries there. I have a EVGA GTX 670 video card in the same system. My reasoning for wanting to upgrade the CPU is to get better gaming performance without spending any extra money. I originally had plans to upgrade the motherboard/cpu/ram/PSU, but things changed (as in, my computing budget) once I found out my Wife and I are pregnant.
It all depends on the motherbd really but q9550 will probably be slightly cheaper of the two. Either one should hit 3.6-3.8 stabile and anything more depends on how good an overclocker your motherbd is.
There still aren't a lot of games out there that really take advantage of quads, although more than there were a few years ago. It's a good move for future proofing purposes, though.
But presently, between your CPU and your video card, I think you generally have a good setup for gaming (at least for most titles) as you are. You could wait until Haswell comes out to see if that reduces pressure on the price of the Yorkfields, but that is a ways off and given your situation that might not be ideal from a timing standpoint.
Well, most games I play my CPU is above 90%. With all settings max in BF3 (for example), I'm getting around 30 to 40fps (with some dips in the 20's). However, I've seen stats with people using the same video card and Quad-core CPU's getting 70+ fps. Although I understand most games don't fully utilize quad-core CPU's, they are definitely better for gaming from what I can tell.
They are definitely better, I agree there, if for no other reason you have two extra cores managing background tasks. The question really is, is are you willing and/or able to spend $100-150 immediately for a Yorkfield? If not, or if it is a purchase that is not critical to the continued enjoyment of your present games, then you may want to hold off on it until Core 2 Quad pricing drops or a more substantial upgrade opportunity presents itself.