The Mac Pro is obviously the fastest computer that you'll get for that money, but you'd also need a good screen for your work, which adds a lot. The Mac Pro may even be a better investment, because you could upgrade it much more than you could with say, an iMac. Then again, it's probably your most costly option, especially if you don't have a good display.
The MacBook Pro is portable, but the screen is a lot smaller than what you'll get with an iMac or Mac Pro. You'll get a lot less power overall. Plus, laptops are not the best for gaming.
I'd say go with the iMac. It takes up less space, and costs less than the Mac Pro, and it has a bigger screen and more power than the MacBook Pro.
I'd get it with the upgraded processor and gfx card, then use whatever is left over to buy RAM from a 3rd party, which will cost much less than Apple RAM.
Note, however, that Photoshop will be really slow, no matter which intel mac you get, because it's under Rosetta.
If you can, go to the local Apple Store and test-drive the Macs there, just to see what you like.