I use a 27" Toshiba for TV gaming. Not a particularly great TV, in fact I'm not very happy with it because it has some geometry problems, but I got it super-cheap a few years ago. I have either an Xbox or Gamecube hooked up to the component inputs, but only one at a time because I only have one set of inputs and no switcher. Turbografx-16 with CD unit is hooked up to one of the composite inputs. PS2, Saturn, SNES, and Dreamcast are hooked up to an S-video switchbox. Whenever I feel like playing N64 or PC-FX I'll hook those up to the last open input on the switchbox, but I hardly ever use those two systems so they're usually not hooked up. For audio, I just use the TV's speakers.
For PC gaming I have a rig I built almost four years ago. Really overdue for an upgrade, but I don't feel like spending the cash atm (I just bought Half-Life 2 though, maybe that will force me to upgrade). I use a 19" Samsung CRT with the PC. I have another Dreamcast that is hooked up to the monitor via VGA box. Finally, I have an XRGB-2+ to use my old Playstation and another Saturn on the monitor. I have to use a switchbox so my PC, DC, and XRGB can all be hooked up to the monitor at once. For audio I have a 5.1 speaker setup, but it's mostly only good for PC games. Trying to route audio from the consoles thru my PC sound card sounds terrible, so I'll usually use headphones with the consoles. I also have a component to VGA adapter that I use with the Xbox and Gamecube sometimes to play on the monitor, but not very often because I have to lug the systems back and forth from my TV (and get them all connected).
I've been wanting to build a home theater setup for awhile now, but I'm waiting for a few things. Mainly, I'm waiting until I buy a new house (probably sometime in the next two years) which also gives me time for TV prices to come down and figure out which of 360/PS3 is going to be the one I want. I don't want to get either until I get an HDTV since it seems like they'd mostly be wasted on an SDTV.