You'll have to change everything except the OS and case really. Even then, Windows 7 would be a good idea. The Thermaltake Xaser III is decent enough.
This is from newegg.com.
AMD Phenom II X4 945 3.0 GHz - $139.99
ASRock 870 Extreme3 - $89.99, $2.99 shipping
OCZ AMD Black Edition 4GB (2 x 2GB) OCZ3BE1600C8LV4GK - $84.99 after rebates
Gigabyte GV-R583UD-1GD Radeon HD 5830 1GB - $169.99 after rebates
SAMSUNG Spinpoint F3 HD103SJ 1 TB 7200 RPM - $74.99
Corsair HX650 - $99.99 after rebates
ASUS DRW-24B1ST - $22.99
Total = $685.92
You're missing a new operating system just so you know. In terms of gaming, you're not missing too much although newer DirectX 10 only games won't work, and you won't be able to play some games in DirectX 11 mode which looks pretty good.
I don't know what monitor you have or what Hard drive....... I used a new machine on an old monitor once and it didn't last long. The opsys put a $100 drain on other components.... and for a video card.... a 5850 won't fit the budget but would run much cooler.... so.... says $702.00