Creative Professional Grade Computer

[JustDraw!]

Inkwood,

I think its going to come down to what your budget is.
I have:

2012 macbook pro, 15 inch, Mid 2012
2.3GHz Intel Core i7
16GB 1600MHz DDR3

and so far its great for my art needs.
In most cases, the better your hard rive speed, memory, and cpu the better your photoshop experience.
If you want an iMac specifically, i say go with the

27inch
3.2GHz quad-core Intel Core i5
Turbo Boost up to 3.6GHz
8GB (two 4GB) memory
1TB hard drive1
NVIDIA GeForce GT 755M with 1GB video memory
http://store.apple.com/us/buy-mac/imac?p...tep=config

And go here to upgrade your memory, which is cheaper than buying it through apple.
http://eshop.macsales.com/shop/memory/iM...2_27/DDR3L

The Other World Computing guys have some cool stuff for Apple products, and I haven't been disappointed yet.

What are your specs on your current macbook pro? Cause maybe you just need more ram or a better hard drive. Yet, if its really old, and you have the grad money any way…

Best of luck in your decision making, I know how difficult it is to spend so much money at once.