Personally I don't game enough to really know. My post was based on the general consensus among gamers (and a developer or two) on a different Mac forum. They pretty consistently recommend the MBP over the MP for serious Mac (or Bootcamp) gaming. For what little gaming I do (highly unlikely to play the latest, greatest version of any game), I can get away with using any new Mac.
If portability is not important, I think the OP would get the best performance per dollar from an iMac, which has the benefits of a dedicated video card, processing power comparable to the MBP, and does not suffer the limitations of a laptop display, which would be more apparent to heavy Photoshop users. It does have the drawbacks of poor expandability compared to the MacPro, and lacks the MBP's ExpressCard slot, which could be used to connect an external drive via eSATA. It does, however, have the FireWire 800 port (not available on the MacBook or the Mac mini), which still offers excellent, albeit inferior to eSATA, bandwidth for external devices.
My guess is that that recommendation is more based upon the fact that the Mac Pro does not really gain you much (if anything) in gaming performance over the MacBook Pro, but costs significantly more...not because the Mac Pro is a poor gaming platform. In otherwords, why spend more than $3000 for a Mac Pro (with monitor) compared to a $2500 for a MBP (you would want the upper end 15" at a minimum for the "beefier" graphics card for gaming purposes) when the cheaper computer will likely do just as well for gaming. This all assumes that gaming is the only purpose or at least primary purpose by a wide margin. Frankly, if that is the case, then I would likely recommend NOT going with a Mac at all (as much as that pains me). You can get MUCH more bang for your buck with a Windoze computer for gaming purposes (or even just get a gaming console like a PS3 or Xbox 360 or Wii).
The whole picture changes is the primary use is for Photoshop...especially, heavily duty, professional Photoshop use. If the OP is using Photoshop on large picture files doing a LOT of filters or other big time manipulations, then a Mac Pro can REALLY pay off. The ability to have butt loads of memory is the first big benefit. But, then there is the tons of cores that you can get...I am pretty sure that Photoshop can fully take advantage of multiple cores. And then there is the ability to drive multiple 30" displays if you want/need.
I would agree that an iMac might be a good choice...if the Photoshop use is not off the scale. The more intensive the Photoshop use, the more likely a Mac Pro might be the better choice. But, if the Photoshop use is light to "average" in level, then an iMac might work rather well (ignoring for the moment the glossy screen which could be a major pain in the rear in some situations). And an iMac will be largely very similar in performance gaming wise to a MBP.
[soapbox] Backup good...no backup bad!! [/soap box]