@whs
I despise your hatred of Linux. :)
@smorton
In your case, I would go for a system off the shelf, or a customized one from Dell or a similar company. I would definitely take advantage of the Windows 7 upgrade, because Vista... yucky :)
In all seriousness though, Windows 7 will be much better. Vista is in many ways the black sheep of the Windows family. (Windows ME is that part of the Windows family that never gets invited to reunions, has been airbrushed out of all the family pictures, etc.) For some, as whs will attest to, it gives no issues, for others, like myself, it is a headache. I know this isn't a "hate Vista" topic, but it took me about 2 hours to get my girlfriends Vista computer to recognize the network correctly at college. It kept thinking there were three connections (thought only one cord was connected and the wireless was off), and none of those connections could see "The Internet" (they were all treated as LAN). Her roommates XP computer took 2 seconds to connect.
Back to the topic at hand though. Even if you plan on doing Photoshop, you can upgrade RAM and a video card at a later date. The "beast" computer I was referring to literally would be for EXTREMELY INTENSIVE work, mainly CAD and Video Editing. For the non professional, RAM and Video Card (as whs stated), if fine for Photoshop. In fact, for a professional using Photoshop, the only other thing I could see them needing would be a faster processor (though I cannot confirm that).
I have to agree with whs that a pre-built will suit your needs, and you get the benefit of the entire computer having a warranty in one place.
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@whs
I mean no offense with the Linux/Vista comments. They are all in good fun.