For mame, the rule of thumb is to:
Get the cheapest new parts you can find.
While a top of the line abit motherboard is great, you can get away with a sale of the week mb at yoru favorite online computer store. They usually give you a deal with an included processor and those are the deals to take and build your system around that.
So an athlon 64 is overkill, while an athlon xp would be just right. Again, get a package deal, see above.
Also get a good video card. Mame might not use it, but every other emulator does, especially 3d ones. Also good modern video cards support hardware rotation, which is quite useful as well as a superior quality tv out.
With ram just get a lot. More than a gig is wasteful though. Again, super spiffy top of the line ram isn't necessary, just get what's on sale (at the right buss speed of course).
A good mame pc needs about a 2 gig processor give or take 500 mhz and 512 megs of ram with at least a radeon but probably a radeon 8000 or higher.
Fancy harddrives are a waste of money as all emulators load the entire game into memory. Get what's on sale and make sure it has plenty of storage capacity.
The powersupply is about right. All the mame accessories you tend to plug in eats up the wattage. And with any computer, the powersupply is the ONLY thing you can't get cheap with.
And btw, in the computing world, name brands mean absolutely nothing. Abit may be the mb of choice for the gamer elite, but unless you are running doom 3 at 3200 x 2400 in full detail or something crazy like that, you aren't going to see any diference between it and a comparable generic brand. As a matter of fact, I make it a point NOT to buy anything abit. Their brand name is half of the cost.