Alright so I've got a MacBook Pro (2008) and to my disappointment I couldn't even run the Crysis 2 demo. Crysis and Warhead ran okay on all medium with no AA. But my GPU isn't supported for Crysis 2 so no sequel for me!
So my mission is: what is the best PC I can make for under $600? A PC that could churn out a solid frame rate.
So what would your rig look like?
Suggestions and links to say places like Amazon.com with prices and the likes would be great.

I also have a MacBook Pro from 2008. When I ran the demo on it for the first time it said my video card (GeForce 8600M) wasn't supported, but it worked anyway. But it doesn't run well enough on Low settings to be able to play the multiplayer component.

Use Newegg.

GTX 460: http://www.newegg.com/Product/Product.aspx?Item=N82E16814130562
AMD Quad core @ 3.2GHz - http://www.newegg.com/Product/Product.aspx?Item=N82E16819103808
4GB DDR3: http://www.newegg.com/Product/Product.aspx?Item=N82E16820231277&cm_re=DDR3_4GB-_-20-231-277-_-Product
MB: http://www.newegg.com/Product/Product.aspx?Item=N82E16813130558
PSU: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016&cm_re=550w_power_supply-_-17-371-016-_-Product
250GB HDD: http://www.newegg.com/Product/Product.aspx?Item=N82E16822148451
Case: http://www.newegg.com/Product/Product.aspx?Item=N82E16811119068
DVD Drive: http://www.newegg.com/Product/Product.aspx?Item=N82E16827135204
Windows: http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754&cm_re=windows_7_64_bit-_-32-116-754-_-Product

Total Damage: 757$ est. Shipping = 20$ = 777$ (Lucky! =P)

Although this is 177$ above your limit, this system will give you some killer framerates and offer some nice future-proofing. You can definitely find something for cheaper, but this is a good bang for the buck.

1. LOLAPPLE for gaming.
2. The 8600 is below the minimum reqs. Obviously it won't be playable on it.

1AMD Athlon II X4 640 Propus 3.0GHz Socket AM3 95W Quad-Core Desktop Processor ADX640WFGMBOX
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2. ASUS M4A87TD EVO AM3 AMD 870 SATA 6Gb/s USB 3.0 ATX AMD Motherboard

CPU MOBO COMBO for $183

CORSAIR XMS3 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model CMX4GX3M2A1600C9
$50
Digital Creativity Suite Bundle w/ Registration
EVGA GeForce GTX 460 (Fermi) SSC+ 01G-P3-1380-KR Video Card
$189
Western Digital Caviar Blue WD5000AAKX 500GB 7200 RPM SATA 6.0Gb/s 3.5" Internal Hard Drive
$40
Free shipping w/ promo code EMCKGJK22, ends 3/16
LG DVD Burner - Bulk Black SATA Model GH24NS50 - OEM
$18
Case from thrift store.........$5 (ugly but who cares)
Antec BP550 Plus 550W Continuous Power Power Supply $50


Totals around $535 Where you get windows, we won't ask so that you can stay under budget...but I suggest you buy it cause it's only $100