All right, so it is a a gaming PC. I don't know anybody who'd buy a GTX 260 for Microsoft Office
A Q9550 would beat that 3GHZ Q6600, but not by enough to be worth $300. So forget CPU upgrades this year and next.
More than 4 GB of RAM would help very little in games. http://www.bit-tech.net/hardware/2008/07/08/is-more-memory-better/1
Replacing the video card would be silly too, since the GTX 260 is still one of the best. You'd spend $500 for a HD 4870X2 or $450 for a GTX 280 but the improvement wouldn't be worth anywhere near that much.
Adding a second GTX 260 is probably the best thing you can do, considering your SLI motherboard. It would make a difference in Crysis at 1680x1050. Not really needed elsewhere IMO. However, the 650TX is not powerful enough for all that. If you like this idea, get an Antec 850W or a Corsair 1000W. Check this out:http://www.slizone.com/object/slizone_build_psu.html
Maybe you should just get some nicer speakers, or more HDD space.
ViewSonic X Series VX2255wmb Glossy Black 22" 3ms Widescreen LCD Monitor w/Webcam - Retail, $250http://www.newegg.com/Product/Product.aspx?Item=N82E16824116078
SAMSUNG 2253BW Black 22" 2ms(GTG) DVI Widescreen LCD Monitor - Retail, $240http://www.newegg.com/Product/Product.aspx?Item=N82E16824001268
ASUS VW222U Black 22" 2ms(GTG) Widescreen LCD Monitor with HDCP support - Retail, $180http://www.newegg.com/Product/Product.aspx?Item=N82E16824236028
Try to buy a monitor from a brick-and-mortar after you see how it feels and check for dead pixels.