Heres some suggestions and links to Amazon where I found the best prices:\
Corsair 850 Watt power supply (you may not need one this big, but here it is for reference)http://amzn.to/lHdtiw
Sapphire HD5850 for $149. These will get you about 350 MHash with proper tweaking.http://amzn.to/jfFMRP
2 Gig Kingston Ram. Will be PLENTY for mining, and its cheap at $21 buckshttp://amzn.to/mom2Wr
AMD Sempron 140. Amazon had the best price I could find on it at $37http://amzn.to/ilK3X3
This is the case I use. Its an Antec 300, and has plenty of room for GPUS and is well ventilated with the included dual 120mm fans.http://amzn.to/lgHjr4
Hope that helps!