I was in your situation not so long ago and decided on the Q9550. A stepup in socket, cpu and therefore also DDR3 ram was too expensive - so I decided to go for the quad core and 8Gb of memory, so my system could keep up with i5 i7 systems. So far I've been really happy with my choice.
I was at a LAN party last weekend, and my system owned most of the i5 machines there, and almost kept up with the two i7 DDR3 systems present at the LAN.
More and more games are written to use multicore cpu's - if you look at BFBC2 for example, it really thrives on triple- or quadcore setups.
Well that's my opinion
Btw - if you decide to go with the Q9550 then your graphics card will bottleneck the system.