From my limited experience there isn't much difference building an awesome gaming machine and a photo/video editing machine.
Well, the way I think of it...the needs really aren't that similar (but the end product is probably similar for either use).
For gaming, you need something that can quickly process and display a constant stream of information. You need the display be able to keep up with the constantly changing game (or whatever) that is on the screen. I believe this is where you want a high end video card.
Photo editing, on the other hand, isn't about a rapidly changing display. Sure, you don't want a display lag when making changes to your image, but it's not a constant, ongoing stream of changes. And before a few years ago, photo editing didn't really tax the video card at all. The latest versions of Photoshop have started to use the RAM available in the video card, but that may only be for things like smooth transitions when zooming in or out.
For photo editing, the CPU and the RAM are much more important than the video card....but with gaming, the video card is a big piece of the puzzle.
I was looking to get a nice desktop to look at / edit / etc etc with my pics. The one thing i wanted was a TRUE HD display. Allowing me to see at least 1920x1080. This is going to give you the best idea of how your pics look. Above all else you really want to look at your monitor (what do you have now ?) Something with a fast display - maybe LED - full HD - Nice colors? That is your key component IMO. A nice GPU and Ram will be great if you are editing photos but if you are looking a a distorted photo how can you evening begin to edit that... An option to look at and What I went with is a iMac - Quality machines and their 21.5in goes for ~$1000 (A little over your budget) You can custom build them with a little more umphff but right out of the box they are VERY nice and most importantly... they have a BEAUTIFUL display.
Don't confuse a 'beautiful' display and one that is ideal for photo editing. As I mentioned above, most monitors these days use TN technology, which is good for general computing, gaming, video etc. They can be pretty bright and can look great. But the problem for photo editing, is that they are not capable of displaying a wide range of colors. If I remember correctly, they can't even fully display sRGB, which has a small color gamut. Something like Adobe RGB or Prophoto RGB is way above that.
Also, there is the issue of calibration. For accurate photo editing, you need to have an accurate display and to do that, you need a calibration system (device plus software). You can calibrate a TN screen, but it's not going to make the display perfect. IPS screens are much better suited for photo editing, and while they are more expensive, some of them have come down in price a lot, in the last few years.
Yet another issue, is where the calibration is applied. A typical (lower end) calibration system will crate a display profile and load that into the computer's video card. So the card is trying to interpret the display info and tweak it for accurate display on the monitor. That's not a perfect system. High end 'photo editing' monitors will have their own Look Up Table, so the calibration profile is loaded right into the monitor, which is a better system for getting accurate display.
That's my understanding anyway. If you want a better explanation, find Garbz.