With a 3.0GHz E6550, you won't see any significant improvement with either if all you're doing is playing games. In multi-threaded applications and multitasking, the Q9550 is much faster.
However, both are overpriced online. If you live next to a Microcenter, get the Q9550 for an amazing $170. If you don't, see if you can get a Q6600 off ebay and overclock it (they go for ~$110 used $150 new) or a used quad core.
Otherwise, consider saving up for a new computer down the road in 2-3 years.