what issues will I face if I switch from WindowXP to an iMac?
I use CS5 can I move it to the new platform?
Will old hardrives work?
Will Photoshop files open?
Adobe are very helpful with respect to changing OS, but you will need a new serial number, so there's some deactivation and form filling involved, but it doesn't take long and you can get on with working using a trial version whilst the tedious admin gets completed. If you plan to upgrade to CS6, I think Adobe plan to make the serial numbers cross platform from that release on, so it should be easier soon.
External hard drives on USB should be no problem, but I'm not sure how well the standard PC trick on moving physical drives into a new case as extra drives will work on a Mac Pro.
Files should open without problems.
The two more serious issues are;
1. Support for older specific hardware, scanners, printers, spectros etc. Best to do some searching to check anything important will work as expected.
2. Getting to used to a different way of working. There's so little difference between the two OSs in day to day use it's no big deal to switch, but there is a different philosophy behind the way everything is managed. You may find that a refreshing change or really annoying.
I've got both, but much prefer Windows 7 and think it offers better value for money for me.