well if you're not planning on buying monitor/OS/peripherals you could get a decent setup.
power supply: its important to get a high quality power supply, usually brands like antec and OCZ make good power supplies. People usually skimp out on this, but its actually one of the most important parts.
motherboard: i don't know much about brands, but get one that has the specs you need, pci-express video interface (newer than AGP), SATA, socket 989 for AMD or LGA 775 for intel (for gaming, AMD is better).
cpu: higher speed = better. some have dual core, but on your budget you won't afford it, and i don't think you'd need it. for gaming, i suggest AMD.
ram: 1 gig (2 * 512 for dual channel is best) is the basic for good performance. DDR400 if you're gonna stick w/ AMD. more expensive rams have lower timings, but you won't worry about that on your budget.
hard drive: 8mb cache is basic these days. 100gb minimum if you're gonna be installing lots of games.
case: pick the one with good air flow, and good looks.
video card: good budget cards these days are the Nvidia 6800gs, or if you know how to overclock, the ATI X800GTO2.
this is the setup i would probably get, although its slightly over your budget (so take off where you think you don't need)
all from newegg
ASUS A8V Socket 939 VIA K8T800 Pro ATX AMD Motherboard - Retail
AMD Athlon 64 3200+ Venice 1GHz HT Socket 939 Processor Model ADA3200BPBOX - Retail
CORSAIR ValueSelect 1GB (2 x 512MB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Unbuffered Dual Channel Kit System Memory Model VS1GBKIT400 - Retail
eVGA 256-P2-N386-AX Geforce 6800GS 256MB GDDR3 PCI Express x16 Video Card - Retail
$15.00 Mail-in Rebate
Seagate Barracuda 7200.9 SATA NCQ 3Gb/s ST3160812AS 160GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Antec SmartPower 2.0 SP-450 ATX12V 450W Power Supply - Retail
RAIDMAX Astro ATX-268WB Black 0.7mm Japanese SECC ATX Mid Tower Computer Case - Retail
total : $653.96
and you'd also have to add tax and shipping.. shipping is around 40 bucks. and you get a $15 mail in rebate for the vid card.
if you need to buy dvd-writer, floppy drive, monitor, mouse, keyboard, speakers, then you'll have to seriously downgrade from the specs i gave you.. (i'd strive to keep it like that tho..)
if anyone else has better suggestions plz feel free to correct my recomendations.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius