You have to careful here now. If you are thinking about going to DOS, you'll probably need to get a new video card as well as a new sound card depending on whats already in there. The integrated audio may not work too well. What type of monitor are you using? Are you using an arcade monitor (including arcade VGA?) Then you'll probably need a different video card especially if you are planning on running ADVMame etc...
I bought a nice 500 MHZ Pentium III computer on MAME for my tastes which had a 20GB harddrive, 128MB memory, floppy drive, tower case, and ATI Pro video card. Works great except I did have to upgrade the sound card and video card. The computer only cost me about $175.00
So, check ebay to. For most MAME games (including all the classic 80's early 90's), a 450-500MHZ pentium iii should be fine.