Not much at all as long as MACH3 is ALL it does.
32-bit version of Windows 2000, Windows XP, Windows Vista, or Windows 7 Operating System (64-bit will not work)
Non-integrated Video Card with 32MB RAM
Basic Computer Skills (ability to copy/rename files, browse directories, etc)
ArtSoft USA - Home of Mach3 and LazyCam
You can get a perfectly fine computer for MACH use off eBay for $200 tops, including shipping. There are likely $50 options out there as well.