I am one of the biggest AMD fans out there. (I am currently running 4 AMD systems at home. But, let me be the first to tell you, that you would be better off either waiting until the AM2+ chips come out, or if you can't wait, get a Core 2 Duo. The price/performance right now is pretty scued. You can get a E6600 for about $200 and it beats a AMD 6000 in almost all tests.
Not to mention the fact that they also overclock something fierce. Why spend $500 alone on the case, mobo, and cpu. When you can spend $300 for a 680i mobo and CPU. And, then have $200 left for a better case. If you want to stay with the small form factor, get a mATX mobo, and buy a X-QPack case.
Try this combo:
ASUS P5B-VM mATX mobo $114.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131043
Intel Core 2 Duo E6600 $220.90
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115003
Apevia X-Qpack2 mATX case w/500w PSU $89.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16811144140
Total = $425.88
Look up some reviews, and you'll see, sadly, how much an E6600 will stomp a AMD 5200.