Since you already have 8GB of DDR2, and Q9550, your videocard is still the most limiting component for gaming, especially given the resolution of your monitor. My advice is to overclock the CPU to 3.4ghz+. This should be very easy since the Q9550 has a high multiplier of 8.5x. Your motherboard has a P35 chipset which means it should handle 400 FSB with a 45nm quad-core without problems. Therefore, at minimum you should be able to get 3.4ghz out of your processor (and likely with barely any voltage increase, if at all). You will probably need a FSB voltage bump of +0.1V, and MCH voltage bump of +0.1v. Don't forget to lock the PCI Express frequency at 100, and make sure to set the lowest CPU:RAM ratio when overclocking so that you aren't being limited by your RAM.
Just pick up a good aftermarket cooler, such as the Cooler Master Hyper 212+ for
$30.
I still think you can ride out the wave with the Q9550 @ 3.4ghz-3.5ghz until next year when Ivy Bridge launches. The top cards are almost
2x faster at higher resolutions than the 4890 in more modern games.
The games where the Q9550 may be lacking would be Civilization 5, Starcraft 2, etc. But those games don't really need 60 fps to begin with. Sure a Sandy Bridge system would be faster (but is it worth a $150 mobo + $225 CPU + $50 for RAM?), if you manage to overclock the CPU, you'll be
more GPU limited.
Although I should mention that there are many users who would love to have your Q9550 on the used market. A quick look at EBay shows that used
Q9550 sell for astronomical prices. So perhaps that's also something you should consider since that CPU is in high demand for anyone still chugging along on a dual-core processor on Socket 775.