-Ok i am going to assume you have windows xp-pro, which is still a ok os, but it is limited to only 3gb and dx9.0c.
http://www.techpowerup.com/120231/Phenom_II_X4_960T__Zosma__Unlock_to_Phenom_II_X6.html
http://cyberneoworld.com/zosma-turbo-core-equipped-phenom-ii-x4-chips/
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103995 AMD Phenom II X4 960T Zosma 3.0GHz
(pasted below)
Turbo core, 5 or 6 core on the cheap, BE unlocked multi up to 3.4ghz (17X multi max)
Rosewill FBM-01 Dual Fans MicroATX Mini Tower Computer Case
Item #: N82E16811147123
$29.99
Recertified: Seagate Barracuda ES.2 ST3750330NS 750GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive (don't know if that is good, maybe a new 500 is better for $10 more?)
Item #: N82E16822148824
$34.99
ASUS M5A78L-M LX AM3+ AMD 760G Micro ATX AMD Motherboard
Item #: N82E16813131728
$64.99
Recertified: PNY RVCGGTX560TXXB-OC GeForce GTX 560 Ti (Fermi) 1GB GDDR5 PCI Express 2.0 x16 HDCP Ready Video Card (if not exchange it for a new hd-6870, well it's worth a try to get a 560ti)
Item #: N82E16814133417
-$80.00 Instant
$259.99
$179.99
KINGWIN ABT-650MM 650W ATX 12V v2.2/ BTX SLI Ready CrossFire Ready Power Supply
Item #: N82E16817121084 (should be fine with a single card, but with a 70%@100% rating it's is not so great so expect a actual 550watt output, i doubt it can power crossfire/sli)
$49.99 ( corsair, antec, silverstone is better if you can find one on sale)
Pareema 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Desktop Memory Model MD313C80809L2
Item #: N82E16820576003
$23.49 (maybe a extra $5 to go to corsair memory since the cheaper brands are sometimes taking a chance)
AMD Phenom II X4 960T Zosma 3.0GHz Socket AM3 95W Quad-Core Desktop Processor HD96ZTWFGRBOX
Item #: N82E16819103995
$124.99
Subtotal: $508.43 (add dvdrw if u need one unless u can reuse your current one, if not add $20)
http://www.newegg.com/Product/Product.aspx?Item=N82E16811147123
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148824
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131728
http://www.newegg.com/Product/Product.aspx?Item=N82E16814133417
http://www.newegg.com/Product/Product.aspx?Item=N82E16817121084
http://www.newegg.com/Product/Product.aspx?Item=N82E16820576003
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103995
-Well anyways under $550 makes some compromises, risk taking to get a fast system. Can u reuse a case? dvdrw?
It could save enough to get some better parts.