I went from a dual e8500 to a Q9550 and noticed a huge difference with games that made use of multiple cores, ie: BF Bad Company 2, Some others.
For that jump it made sense, And its not even over clocked. I found the dual core would stutter the gameplay and it was frustrating, so i said heck with it.
I purchased the Q9550 new before it was discontinued for 280. Looking back i could have probably got a AMD 6 core - T1090 and AM3 Motherboard for probably 300ish. The 6 core i mentioned is virtually the same as my Q9550, which is a solid chip. I chose the lazy route.
IMO, worth the upgrade to a available quad.