What you need?
-a case   its on your taste
-a motherboard   if youre buying an AMD one, it MUST be AM3+ compatible, with place to put on 4 ram sticks    i use a Gigabyte one, works well, heard the Asrock ones are good too
-a PSU      a quality 500W will be enough for everything. I would go for a chieftec.
-CPU    quad core phenomIIs work very well, can be overclocked like crazy, cheap, upgradable to the new coming bulldozer
-RAM    DDR3 is a must have, at least 4GB. Dont buy noname stuff, the Kingstons are cheap and good quality. DDR3 1600mhz is the way to go
-HDD   is you only wanna run AH, dont spend so much on this, that Western Digital one may serve you well
-GPU   well, there are many choices. If you want to run AH with all the eye candy on, you may need a radeon hd 6850. If you remove the shadows, a radeon 6570 will get the job done.
-DVD rom     dont spend too much on it unless you wanna write blu-ray films
-OS    it can be expensive, but free is you already have one
Its all together about 550-650 bux here (EU), depending on your video card choice. Its a decent, upgradable machine.