i7 920 (D0 stepping, if you have a choice) ASUS P6T SE (or The P6T Deluxe V2 if you want SLI support and/or improved heatsinks. FYI: both versions come with Crossfire support) 3x2GB DDR3 1333 or 1600 (I recommend these: http://www.newegg.com/Product/Product.aspx?Item=N82E16820231230) Corsair 750TX Samsung Spinpoint F3 500GB (Consider getting two and setting them up in Raid 0) Radeon HD 5850 Antec 900 or 1200
TOTAL: roughly $1,200-1,300 USD depending on local prices and exactly what you get.
If you're going to overclock, get an aftermarket cooler. I recommend the Noctua SE1366, XIGMATEK Dark Knight, or COOLER MASTER V8.
I also recommend you consider a new monitor that will give you 1920x1080 or 1920x1200 resolution to take full advantage of your system's capabilities.
If you're only running 1680x1050, there's no need to spend your full budget on a computer unless you plan to upgrade monitor resolution at a later date. The only possible exception is if you want to max out Crysis settings at 1680x1050.
Any other game, and you're swatting a fly with a hammer with the box you could build with $1300. You're looking at an i7, Radeon 5870 build that will destroy 1900x1200.
You could easily get away with an i5, Radeon 5850 build for several hundred dollars less at that resolution. Unfortunately, your budget won't permit building a $1300 box AND upgrading your monitor at the same time.