My squad had a fellow squaddie who had a ten year old computer and was dealing with hard times, so we built him a new computer as his old one would hardly run Aces High, and disconnect him every time he hit the VOX button. Here is the parts list for the system itself, which came to just under $438 with shipping. His system averages about 120-130FPS (vsync off for testing) and this is sliders maxed, hires pack, all options on, 1920X1080 resolution, and self shadows (smoothed) at 1024. The only thing turned off is other planes' shadows. This is also running Win XP Home.
For a cheaper processor, it actually keeps up pretty well. I got him a quality motherboard and a reliable power supply. The only thing I may have done differently if I had to do it again would be the case.. it's definitely cheap but it does hold everything together (and had a surprising amount of room for cable management). If you have headphones plugged in the front jack and plug in a USB device in the front jack, then you get a lot of sound interference, but other than that it works fine.
Keep in mind the prices may have changed but it should give you an idea:
Rosewill FB-01 Black ATX Mid Tower Computer Case, come with 1x 80mm Fan
http://www.newegg.com/Product/Product.aspx?Item=N82E16811147090$17.99 Free Shipping
SeaSonic S12II 520 Bronze 520W ATX12V V2.3 / EPS 12V V2.91 80 PLUS BRONZE Certified Active PFC Power Supply
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151094$59.99 + $9.65 Shipping
Seagate Barracuda 7200.12 ST3250318AS 250GB 7200 RPM 8MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148451$34.99 Free Shipping
Link Depot FAN-80-BK 80mm Case Fan
http://www.newegg.com/Product/Product.aspx?Item=N82E16835104002$3.49 Free Shipping
ASUS Black 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 12X DVD-RAM 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM SATA 24X DVD Burner - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16827135204$18.99 Free Shipping
GIGABYTE GV-R577UD-1GD Radeon HD 5770 1GB 128-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125309$139.99 + $7.56 Shipping (Not Including $20 Mail In Rebate)
AMD Athlon II X2 260 Regor 3.2GHz 2 x 1MB L2 Cache Socket AM3 65W Dual-Core Desktop Processor ADX260OCGMBOX
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103873$67.99 Free Shipping
ASUS M4N68T-M AM3 NVIDIA Geforce 7025/nForce 630a Micro ATX AMD Motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131626$49.99 Free Shipping
Mushkin Enhanced Essentials 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel Kit Desktop Memory Model 996586
http://www.newegg.com/Product/Product.aspx?Item=N82E16820146748$59.99 Free Shipping