I built one a few months ago... Intel core i7 Ivy Bridge CPU, GeForce GTX 670 2GB video card, 16GB RAM, 650w power supply, 1TB HDD, and motherboard with an Intel z77 chipset and space for 32GB of RAM. I'll be maxing out the RAM when I can, but an IPS monitor comes first.

With this build, PS CS4 (64 bit) on Windows 7 runs smoothly with decent-sized files (the largest I've worked on so far is 17 x 21 inches at 300ppi, including about 50+ layers, save time ~ 5-20secs). But the 3D stuff in PS and Sculptris, while better than my laptop, is still very slow, clunky and apt to fritz out on-screen and software-crash unexpectedly. Play-dough is less frustrating for 3D reference, but most of my work is 2D and smaller dimensions, and I have a large container of homemade play-dough, so it doesn't matter so much.

Also, if I leave my computer on longer than a day or two (kept on sleep/awake instead of rebooting) the display develops an odd quirk (sometimes a distorted block of pink/green pixels or blackout of the image in PS, sometimes black diagonal flashes in my web browser)... and if I ignore it the computer will eventually blue screen and dump its memory. I'm not tech savvy enough to know why it does those things (if it's a faulty part or just the way it is), but I now shut it down every night and have had no further issues. If anyone can enlighten me, I'd be grateful.

Point being, you may need a stronger build for 3D work than what I've got.