Personally, I don't think there's really such a thing as too much HDD space. My first PC was a Pack-o-hell (Packard Bell) with 2 meg of RAM and 125 meg of HDD space. Of course then, 125 megs of HDD space was so much that I almost never imagined I'd ever use it up.
A couple of quick questions: What are you shooting with, I.E. what is your typical RAW file size? Are you editing several files at a time and/or batch processing? Either way you'll be at the mercy of your available memory. I'm running Intel duo core w/4 gig memory using Photoshop CS3. This configuration runs fine until I have much more than a dozen RAW files open at once. Of course, Photoshop tends to be a resource hog so Lightroom might behave better under the same conditions.
Space and memory are your friends.
As far as Dell, HP, Gateway, etc. I haven't purchased a PC off-the-shelf since the 386 was king.