The upgrade test:
Open the Task Manager (CTRL-ALT-DEL), change your update speed to LOW. observe the CPU performance. Minimize, but do not close.
Close all tasks, make sure nothing major is running (maybe 5% CPU) and open one of your high-end games. Play for 5 minutes, then close and look at your Task Manager.
Have you maxed out one of your cores?
If no cores have run up against 100% load then upgrading your CPU will have no effect. Try this on a few other games.
Other:
If you did upgrade, you'd find such a small boost by spending even $150 that you might as well get a Q9550 for more money for some future leeway.
Personally, I'd consider maybe doing a 10% overclock on your cpu if needed and putting my money towards a future system.
You've got a nice system and lots of games run great on it. Starcraft 2 and Diablo 3 would look amazing on it. if you really have that money burning a hole in your pocked, consider a better monitor or better speakers.
M-Audio AV40 speakers. Auzentech X-Fi 7.1 Prelude. (if you can, hold out for an OLED monitor in 2010/11. Drool)
(I'm fed up and seriously considering keeping my current system. but transition to a next-gen PS4 or XBox if they do what i want. If the games will install to the hard drive, and we see some strategy etc with Keyboard/Mouse support and a few good games you can colour me gone from this madhouse of upgrading, troubleshooting, and tweaking. Diablo 3 and Starcraft 2 on the PS4? Yes, please..
It's worth noting that the next versions of the PS4 and XBox out likely in 2011 will now have enough power to do full antialiasing on top of looking better and more improved Physics etc. i love the idea that they will tweak a game to a single machine. This is going to be the start of the end for high-end PC gaming. Add in web surfing and e-mail and I'll be soon running out of reasons to own a new PC.
You may think, yeah PC gaming will be around for a long, long time. Not high-end though. The cost of making these games is increasing. The extra issues in supporting the PC are pulling game developers away. I just do NOT see what the PC industry can do to get people from continuing to migrate once the new consoles arrive)