The X6 is a good choice; I'd recommend making sure you get one of the AM3+ motherboards, though. That way you would be ready for one of the Bulldozer 8 core chips soon to be released. And the board would support any of the AM3 socket chips, from the Athlon II X4 up to the Phenom II X6. As long as its a full ATX form-factor, it should have 4 SIMM slots. Then it would be up to the memory controller on the processor as to how much RAM it will support.

 

The only motherboards I have seen that normally support more than 16 gigs of ram have been server boards, and they only support the Opteron series. But with the controller on the processor, and the larger array the AM3+ socket has, it may support more in the future.