Hatchet: I'm somewhat in your position (first time Mac owner, who hasn't purchased yet and I use Photoshop and Lightroom extensively), but for me the only way to go is Mac Pro - I want the convenience of a desktop tower with expandability for the hard disks (up to 4), and lots of memory (I plan on getting at least 5-6 gigs total to start.)
If I wasn't so keen on having a lot of hard disk space (I'm currently using close to 600 gigs on 3 separate hard drives in my PC), I would probably just stick with an iMac - they look great and perform wonderfully (and can go up to a respectable 4 gigs of RAM now.) I just don't want to have 2 or 3 external hard drives sitting all over the place 🙂
I'm stuck waiting until January 15th (MacWorld), where they'll hopefully announce the new Mac Pros available - much more power at approximately the same cost as the current lineup (hopefully maybe even slightly cheaper.)
Expensive? You'd better believe it, but I'm intending to keep this machine for a good 5-7 years, and if I really need something sooner (with quad, or probably octo-core Xeons, I seriously doubt it), Macs have a great resale value, so I could always sell and buy a newer model... 🙂
Good luck with your purchase though, I can't wait to finally get away from Windows (thankfully I never went to Vista, I'm still running my XP Pro 🙂)
-Bryan