two 1x to 16x ext cables
amd 130/140/145 sempron
cheapest 1g or 2g ram stick you can buy
up to 4 radeon 5770, 5830, 5850
rosewill 650w, corsair 650w power supply
2gb flash drive with bamt installed
thats the cheapest system I can make your still talking about $600 for that with 4 5850 cards
which will make you about $5 a day mining btc at current rates after 10c/hr power
I'm not getting into whether thats a worthwhile investment at this point. Just telling you what will work well at the cheapest price point
People have been saying gpu's were a bad investment since about last june and have been wrong so far.
I'd recommend 4gb and then running windows 7 (DONT buy a license key!)
I've been able to run my 2 secondary rigs just fine since october with never activating. Every 60 days I reset the activation timer by going to the command prompt and using "slmgr rearm" command (do this as admin).
windows is SO easy to use compared to linux.. that and I was never able to get bamt to boot up properly, so I went that route...
it's just another option.