Are you looking at a shuttle or full size mobo? i've only built a single 64bit system, unfortunately, but that was enough to know I WANT ONE NOW!!!! lol
Here's a link to the shuttle system he bought. he bought 512 of corsair xms ram and an AMD Athlon 64bit 3200 32/64Bit CPU S754 (Newcastle Core)
Added a Pioneer 108 DVD burner and a 54x CD reader for the install.
The nFORCE firewall was soooo damn easy to configure it was unvelievable. Some of the early builds had a problem with overheating but his is steady at 52C encoding MPEG's with TMPGEnc 3.x
http://www.scan.co.uk/Products/ProductInfo.asp?WebProductID=118007
Thats the only 64 system i have played with but it was enough to convince me the money he paid was well spent!