First off, if you can manage it don't buy a pre-built system, build your own. You can build a system with better components at 2/3rds the cost. Its not as hard as you think. Most stuff from groups like Dell are half a generation to a full generation old anyway and from cut rate manufactures to boot.
Next, some games in MAME are so complex that there is not a PC in the world that can emulate them at full speed. You could drop $10k on the latest and greatest PC in the world and still not play them full speed. Most games will play fine but some newer and/or 3D games wont.
Also, if you want a complete MAME collection 640 GB isn't going to be enough to future proof your system. A current set will take up about 150 GB but that amount will balloon when laser disc games are fully supported.
On a related note. If you want MAME you can get a current build at mamedev.org for free. As for ROMS, well, lets just say you should learn how to use BitTorrent and start looking for some good trackers. Google is a good start. Hope you have broadband, like I said a current complete set is about 150 gigs and thats not counting extra like title shots and cabinet artwork.