A computer has precisely no bearing on image quality, that is down to the photographer and their tools to sort out.
Myself and many others on this forum use Apple's, others use Windows XP. I currently use a 17" MacBook Pro with 2 gigs of RAM and it works well for me processing EOS 30D files. It isn't a cheap computer but I'm dealing with fairly large files on a daily basis and the screen is wonderful. I have a couple of external 250 gig discs in addition to the 100 on board. I use an old PowerMac G4 with 1 gig of RAM and a bunch of discs for storage and printing as Epson don't have a 2100 driver for Intel Macs.
For basic viewing, you could happily use one of the cheaper 13" MacBook models but the small screen isn't ideal for processing files. For RAW processing and Photoshop work, you want as much RAM and screen as you can afford.
HTH
Charly