Thanks for your input, everyone!
I'll definitely go the "build" route, although it may take some time to find the deals you're mentioning!  I'll be patient.  :-)
As far as Steam goes, you're totally right, it may be smart not to go that route.  There are only a few games that I'm interested in, and it won't be a dealkiller if I can't include them.  I'll stick with TIE Fighter or Freespace 2 if I want a good space sim, and I'll save the Steam stuff for my actual PC.
I didn't think that size would be too big of an issue since I'm putting the parts into the cabinet individually, but I guess that it all still has to fit in a relatively small area!  I'll keep that in mind while shopping.
Again, thanks so much for the help!  Time to do some searching!  ;-)
Brian