Alright,
XP is very compatible with just about everything. The down sides are you're prone to frequent crashes and viruses, and it's pretty much dead technology. Microsoft is going to wipe it off the map entirely in a year or so. But you can manipulate it, both hardware and program wise very easily. I would steer clear of brands like dell (especially) and hp (more espcially). Have a nice white box built if you go this route.
Macs are less compatible. You'll have a harder time finding programs and hardware. It will also be more expensive to upgrade. The upside is, they get far less viruses, and crash less frequently, generally. They are prone to crashing though. Other than that, they're just intel boards now a days, so if the os is more comfortable for you, and you don't plan on changing anything, not a bad choice. They aren't better, just different.
Vista is an inordinatly heavy OS with not a ton to offer in terms of graphic design. In all honesty, unless you bought your computer with a 10ghz processor and 13mb ram, vista will slow you down. It's still very buggy at this point, too. But sp1 supposedly fixed some of that?
I prefer a Linux OS, because if it's good enough for google, youtube, and the us airforce, it's good enough for me. There is some trouble with devices in linux, but most can be overcome with a windows emulator. It's less prone to crashing, bullet proof against viruses, and free support and upgrades for the rest of your life. Can't be beat, except it hates my printer. And I think the brand new corel products won't work on it. Someone must have knocked on corel's door with a handful of cash.