the reason also that macs are popular among graphics designers is because the GDI(Graphics Display Interface) is much more efficent in comparison to WIndows. This gives the perception that Macs outspeed PC's in performance. I think the new Apple G5 IS going to give older PCs a run for their money though! I have a feeling the Mac OS GDI is based loosely around Post Script.
Longhorn (not yet released) will feature a new GDI which is good becuase it hasn't been updated for a considerable amount of time however it will require a good amount of hardware muscle and bugs still need to be ironed out for example screen overlay is not supported at present(which is used in DVD playback on PC's).
On the Linux front, it is still relatively new and in my opinion 'raw', if you have used windows for a long time it will feel strange. the most difficult thing I found was getting used to the way files are stored on a *unix machine. If you are a confident computer user you can afford to get a slightly older distro like Redhat 8 or Suse. However, for the easiest transistion I have read that the most recent version of Suse Linux(http://www.amazon.co.uk/exec/obidos/ASIN/B00008XYHO/ref=sr_aps_software_1_2/202-7997556-6454254) is good for people new to Linux. But linux does have advantages due to most linux software being Open Source/GPL(FREE) even advanced server tools! Just to get you started, if the server is going to supply a small office network with Windows PC's you need to investigate Samba. The link to Suse Personal is to show youo a product that could get you ready for Linux, I don't know whether it is of a strong enough calibre for a server though.