I would suggest a $400 Dell then adding a 3850 and an extra stick of ram but that would push you up to around $600. But that $600 would give you a fairly acceptable gaming rig. If you want to play at console graphics levels/frame rates $400 could do it. If you are willing to hit FS:FT or hit the pawnshops and buy used. It's usually cheaper and easier to bring a 1-2 year old pc up to fairly good gaming levels with $250 or so than it is to build from nothing with only $400.
Ok, OP, please help us help you by answering the following questions:
- What parts do you need for that budget? Hard drive? RAM? PSU? CPU? etc. Please be as specific as possible. Do NOT say just the tower/PC as the tower can comprise of many different parts that you may or may not need.
- Will you be overclocking?
Though so far,HardwareGuru's list is pretty good for a start.
$66 - Antec NSK4480 with EarthWatts 380W PSU $60 - AMD X2 4000+ AM2 (free shipping) $49 - BIOSTAR NF520-A2 $40 - Patriot 2GB DDR2 800 (single stick) $23 - Lite-On DH-20A4P-04 IDE DVD burner (free shipping) $50 - WD Caviar SE 160GB SATA HDD $90 - XFX 8600GT 256MB ($20 mail-in rebate)
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$378 - Subtotal (not including shipping, taxes, or rebates)
I'm going to tell you right now -- shipping will push this total over $400. And even though you can play games with the 8600GT, the in-game visual settings may not be to your liking.
Again, either raise your budget or consider getting some of your parts used. You're not going to get a good gaming experience for $400, especially when the video card takes up almost 25% of your budget.
edit: go with HardwareGuru's idea of getting the $20 case and earthwatts 380w psu separately to save money. if two dollars is worth the .2ghz difference then drop to the e2140. getting a 160gb harddrive would also shed off about $30.