If i5 is on LGA775, then it might work, but if its not then its probably too far off to be within range (p55 would have to be out.)
Quad cores are only going to get better as time goes on from a software standpoint, remember when everyone said you were better off getting a single core CPU for gaming?
At this point I personally wouldent buy anything over a low end Q9xxx, simply because of the price of the 920. I plan on using a Q6600 for my next rig, but I'm really cheap and all.
Or, you can save up and get a i7, but even on a game that works well on a quad, it probably wont have that much of an advantage over a C2Q because of that L2 cashe. If games start supporting SSE4.2 then it will be diffrent though....