I have a Thermaltake Armor LCS case, equipped with a fairly standard pump, reservoir, and 2 x 120mm fan radiator, as comes with the case. Currently I am using it to cool my moderately overclocked (2.4 --> 3.0 GHz) E6600. I am looking to buy two new graphics cards, probably 4870s, and watercool one (or both...), but I suspect that my radiator is not large enough - would it be able to handle one, or even two graphics cards being added to the loop? All help is appreciated, I am no expert on watercooling!

Yes, you will be fine with a 2x120mm radiator for at least one card. Two, probably OK as well, but you might be pushing it depending on which cards you get. Also, what pump, radiator, water blocks, etc are you using? This is a more important detail than most beginners think.

What are your waterloop components (GPU blocks to be used as well?)
What cards are you going to use?

As long as you stay with the e6600, you should be ok.

If you move to a quad, then you do not have enough.

I currently run a Q9450 @ 3.4, with two 8800 GTX's in the loop. I had 2 x 120 rads, and all was fine with my e6750. Added the Q, and had to add a third radiator.

I was very suprised the Qaud pushed the temps up more and faster than the gpu's did.

Just be sure the fresh cooler water goes cpu-gpu-rad, and you should be ok. You might add a resevoir for more thermal capacity, which will help keep temps down.

I agree; the Quads get very hot, especially when you OC them. I noticed 3-4C at idle when I went from 3.4 to 4.0. I added a big, passive radiator to the side of my case along with 2 custom acrylic resevoirs that feed into each other. Works really well, and I can feel the side rad get warm as I game a while. Quad doesn't go over 46C load and my 9800GTX doesn't see over 45C load.