If buying a desktop, buy at the upper end of the desktops from HP, Compaq, VAIO or whatever you choose. Lower end desktops are for very general use, upper end are adaptable if not already set up to go gaming.
128 MB Vid. card is plenty, but make sure it is DX9 compliant, that means a relatively newer card, not the older ones.
1 GIG System RAM is a must in my opinion. 512 MB is barely enough if using XP. It is enough, but barely.......and believe me, you are going to get 1GIG even if you start off with 512, so why not do it now!
It would also be in your interest to try and get one that already has a GOOD Sound card already installed in it. If the desktop comes with a good set of speakers, that's a plus for you, if not, look into those too.
Now remember, some of those screenshots you saw are coming off of higher resolution monitors, and probably a good portion of them off LCDs or Plasmas. So if you are getting a CRT(big box) you won't get your moneys worth from the graphics card or system. It will look good with a CRT, but man the difference with an LCD blew me away.
Just things to consider!! Good luck and happy shopping!

Systemax Sabre Ultimate gaming rig.
I got all the goodies and all the power I need to run anything thrown at me for the next two months!