#1) Never buy a Celeron if you're wanting to play hard-core games... Athlons or true Pentium-4s are the way to go.
#2) Always get full-ATX MBs so you have more then two or three PCI slots
#3) So what were you saving the last PCI slot for? It's only useful if you're using it, go ahead and put that last slot to use!
#4) If you're really hard-up for PCI slots, you could get an EXTERNAL TV-tuner (via USB 2.0), but of course that costs more then a PCI solution and adds extra junk outside your case.
Having said that, the 6600GT will probably not run at its full potential on a Celeron system... this is not to say a 6600GT is a bad purchase, because it WILL help your system out to play Doom3, and it WILL port-over to a newer MB/System if you so choose later to upgrade to a faster system (so long as you still have an AGP system). You may not get the best mileage out of a 6600GT combined with a Celeron, but it's definately going to be a big improvement over what you have now.
If I were you, go ahead and get the 6600GT and see how you like it. If it's not enough, you can always upgrade the CPU later (and/or get a new MB/CPU combo). If you've got a Celeron in a mini-ATX system, I'm under the assumption you have on-board graphics? If so, you'll DEFINATELY benefit by getting something like a GeForce6 (you do have an AGP slot, right?).
Of course, if you've got money burning a hole in your pocket, upgrade everything at once, but if you're on a budget, I'd recomend you get a 6600GT and a PCI TV-Tuner.