It's actually cheaper to custom build what you need-assuming you REALLY want a PC and not a Mac.
You want a decent graphics card, but actually graphics aren't the hardest drain by using photoshop. Any decent graphics card will actually handle it.
64-bit operating system will allow you to access more RAM than the 4G max in a 32bit system
AT LEAST a dual core system
6-8G of ram if you're really using Photoshop. Max out your ram. Even if PS isn't using it, you won't be competing between PS and the operating system
The main hard drive doesn't necessarily have to be a giant one. The main should only contain your operating system and programs. A secondary hard drive should contain all of your working files. As well as an external back up hard drive.
Upgrade your power supply. Photoshop is actually a power hog. MANY to most lower end PC's have a very basic power supply and that will crash you sure as sin. I think in my crappy computer I am running a 500W and in my processing computer I am running 750W. I also have a pretty heavy graphics card-which draws, several hard drives, an upgraded sound card (why? I have no clue!) and an upgraded fan, wireless card, CD drive, DVD drive... It's probably more power supply than I need, but at least I have no crashes! I'd guess the 500 would power that system.
NOT windows Vista. XP or 7, but vista is more headaches than it's worth.
Second hard drive that is large enough to store all of your working files, documents, etc. I think mine is 1TB and I don't have it half full-I archive diligently though or I would have it GONE!
A monitor that is appropriate for editing AND A CALIBRATOR! You can skimp a LITTLE on the system components within reason, but the monitor has to be accurate. You could have the fastest gaming machine on the market and it won't do you any good if your monitor isn't capable of displaying your images true to life. Here's a good piece on choosing a monitor and calibration:
www.Clarkvision.com: Choosing and LCD Monitor for Photo Editing