This thread is really upsetting because a lot of people are apparently very misguided. Hopefully I can stop the bleeding a little here. All the MacPro/Powermac/iMac bashing on here is complete hogwash. Please listen, because this is actually accurate information:
1) The iMac is not a pro machine, it's a consumer machine. It's capable of handling a lot of stuff that other consumer machines can't, but in the end it's not built to handle running a lot of pro apps at once. If you need to do that, get a pro machine. There's a reason the iMac is over $1000 cheaper than the MacPro.
2) I have used both Mac and PC, in a professional setting, for over 9 years. I have used numerous models, and numerous versions of many different pro apps (Adobe, Macromedia, Apple, etc) for high level photo editing, layout and design work, web development, etc.
There is absolutely no doubt whatsoever that the Macs outperform the PCs by a WIDE margin in virtually all tests, all day-to-day operations, and all benchmarks. It's indisputable, there are numbers to back it up. My Dual 2.5 G5 absolutely wipes the floor with a brand new Dell XPS 700 (like the one that was described here as cheaper than the new MacPro). The new Dell was brought into my office, and my Dual 2.5 G5 (which is a year old!) ran a CS2 speed test in just over 1 minute... the Dell took nearly 2! And there's plenty more where that came from.
3) Photoshop, as has been pointed out here, is not yet a Universal Binary application, so it's going to run slowly and eat up a LOT of RAM in the process because it's being translated the whole time it's running. This will also slow down your other apps. This will be a non-issue in the Spring when CS3 is released, but in the meantime it still runs effectively enough to work on. If your computer lags to the point of aggravation, or causes you to stop working elsewhere, it's something wrong with your particular unit (or your setup) and not the PM in general.
4) As for the pricing issue, there is no Dell system you can build that has comparable parts to the Mac Pro for the same or better price. It doesn't exist. On the surface you might find one that appears that way, but look at the tech specs and try it out in real world situations. The Dell systems you can build online appear cheaper, but there are a LOT of corners cut and a lot of pieces to the puzzle you don't see. First off, the 2.66GHz Xeon in the MP runs a lot faster than even the 3GHz C2D. It's been tested. Second, the MP has a faster bus than the Dell and the RAM is fully-buffered across the board so it's used much more efficiently.
Yes, the Dell displays are cheaper, but this has been discussed ad nauseum on these forums so I'll suffice it to say that there's room for debate but you wont find more than a handful of people to disagree that Eizo and Apple are in a dogfight for the "best display you can buy" category, and Dell isn't even in the same league except at price point.
Buy a Mac, give it enough RAM to do the jobs you want it to do, and don't for a second think you can find a PC running XP that will do the same job at the same speed. It doesn't exist.