I've played with SME on many platforms, but as it usually needs to run reliably, decent hardware is suggested. I've had excellent experience with Tyan AMD products. Pick any of their server (opteron) series and you will be in great shape. (I have no connection to Tyan, just a very satisfied customer who has spent thousands on their products, and will continue to buy from them.) Remember that SME actaully has pretty low requirements, so you can pick up a lower end MB with opteron with quite a bit of memory for cheap. I like Newegg and Provantage... had great luck with both.
If you are looking for a bit more power, perhaps a dual core would suit you... Dell has come very inexpensive Athlon Dual core boxes for well under $1000... working on one right now.
Hope this helps!