I found that the best way to go is with low power amd builds. Get a cheap (CHEAP) processor, a motherboard (also cheap) that can handle 1-2 powerful cards, and calculate what its power requirements are carefully for the power supply. Buy as little ram for it as you can, same with the hard drive. Use an old dvd player laying around as an optical drive, or again, but a cheap one. the hard drive Im still looking into, as some people can get mining working from just a flash drive.
The 5850 currently is a massively powerful card for mining, 270 mhash/s stock, and Ive got one running overclocked pushing 392 mhash/s. My main miner cost $1000 to build, and puts out 1.2ghash/s. There's a potential to put out as much as 1.6 ghash/s with this one, but Id have to spend $500 on cooling.
What's your build?