I recently purchased a used Q9550 to replace my E8400 since I wanted the extra juice to play BF3 but didn't want to replace the whole system. I spent $170 for it (actually a good deal), minus what I sold my E8400 for ($80), so really only a $90 investment for significantly more power. I guess there are a lot of people out there like me who are in the same situation.
I was actually thinking about doing this myself as I have just about the same system as you but wasn't sure if it was worth it to go quadcore on this old platform. In fact, the only reason was to get BF3 running a little smoother on high settings @ 1920 x 1200. So you are definitely not alone. Still though... I am tempted to wait for Ivy Bridge and get a new CPU/mobo.