Olivia,
While the MAC enthusiasts were first with the use of the photo software and were known to dominate the arts world, there virtually is no longer a difference in the capabilities of Photoshop for MAC or PC and that is the industry standard. The program is identical.
I was in on John's lesson with Robin (and Masaki too)and I agree that to do great photography you really do need to be versed in Photoshop CS3 - but Mac or PC are identical! No MAC required.
There is a new version of Photoshop (CS3) that's been released in Beta testing and the photographers are going crazy over it as well as a new program called Lightroom 1.0.
Even in the photography workshops, which were formerly dominated by MAC users, they are now populated by more PC users than MAC users. That was a real surprise to me, since I was thinking that maybe I would switch my photo work to my Mac Laptop - I have been using a PC for my photography.
PC's are actually less money initially, have more software available for a variety of applications. That being said, I am not recommending that people buy the new VISTA operating software yet, because like anything new, it's been released before it's ready. Even the dual platform MAC's have a few issues that Apple has not addressed, but, it's not widely publicised and hard to find the info about it even from Apple.