I would suggest Mac, if you are focused on photo edition. Mac Book PRo, i5 at least.
For a more afordable computer, try Asus, Sony. DON'T go on HP and Toshiba. Those are the worst in quality. i5 at least, quad core, 4 GB Ram and at least 512 mb Video
I don't want to be negative, but I disagree with most of this.
HP are some of the most reliable systems I've used. Asus cut corners in stupid places to save pennies. I've been inside a lot of their machines, and nearly every component is generic off brand stuff. They are cheaper than the competition, but not by enough to justify what you end up with. And Sony is all about marketability in their computer division. Sony computers are built to be _appealing_, not high performance or reliable. I have a Toshiba laptop from 1999, Pentium II, 192mb ram, 12gb hard drive... to this day runs like the day I got it.
Not sure how your budget will be after software, but if you can get a Xeon processor instead of an i series I would recommend that. They are intel's professional line of processors and offer a few performance advantages. Most significantly though, they are more reliable than the consumer stuff because thermally they are well over spec'd, and they also support error correcting memory. The Core2 and i processors run right at the limit of stability to keep costs down. I'd recommend 8gb ram because ram is cheap. If you're not working in 3d then you will never even need to know how much video ram you have. If the Xeons end up being too costly, i5 and i7 are still very nice chips.