this is a tough one, the Q9550 is a great CPU on a dying (not dead yet) LGA 775 platform. On the other hand, the i7 920 is a fantastic CPU that is more future-proof.
Both are very Overclockable, so not much debate there. OC the Q9550 to 3.6 and you'll see a good increase, not staggering, but definately noticeable. The FPS boost in GTA4 would maybe be in the double digits, (perhaps 8 - 15 FPS increase)
Now, clock-for-clock, the i7 920 is better than the Q9550 and you can easily hit 3.8 on air, but in my opinion, they are both great CPU's. That said, more GPU power is going to yield the best results, but at what expense?...
If it were me and i was in position to upgrade my platform from LGA775 to i7, i would, but that's just me, LGA775 still has a couple good years left but the i7/x58 motherboard will have a longer life.
If you NEED raw power now, go with Q9550 and OC to 3.6 or higher for a nice boost, then add the 2nd 285 for an even nicer boost.
If it is longevity you are looking for, get the i7 and wait a little bit to add another 285 (when you have the extra $$$)