You can definitely get away with a Celeron or Pentium setup, something like this...
Pentium G3220 $69
ASUS H81M-E Motherboard $65
Corsair VS2GB1333D3 2GB (1x2GB) DDR3 $29
Western Digital WD Blue 500GB WD5000AAKX $56
EVGA GeForce GT 610 Silent 1GB $49
That's $268 for your main guts but you will need a case and PSU for it as well. HTPC cases start at around $100 and look just like any other AV equipment or you can get a cheap ass case and hide it. PSU requirements won't be much, 450 watts should be ample.
I included a graphics card there as I find it's better than relying on any onboard solution and that card runs silent and handles all HD audio and video.
Add $50 for a Hauppage remote (the best in my experience), all the essential buttons just work and it makes it more user friendly for the non-techie ones.
I run XBMC (Windows) which just looks at network shares so it won't care if it's looking at a Mac.
Happy to offer any advice where I can