If you bore me, I wouldn't reply to the thread. I'm referring to the fact that the last TEC discussion got a little heated.
Watercooling is generally for people who are after lower temps than aircooling, so they generally are big.
However, depending on your mATX box, you should be able to fit everything internally. You will NOT be able to get the lowest temps possible, and it will get pretty noisy because the TEC generates a heck lot of heat. To get rid of that heat, AND fit all your watercooling components into an mATX box, you'll have to put in a high cfm 120mm fan. Even then, I got doubts on how well that will work out.
I'm talking about you being able to get your hands on a 320W TEC or similar (225W on 12V). If you go for anything much lower than that, you might as well stick with just watercooling. Don't forget that TECs generate their own heat, as well as sucking up the heat from your CPU. This is why low powered, 50W TECs which are generally in complete TEC systems don't work all too well.
As for the watercooling part, you can get pretty much any WC system at all. The reservoir just has to be a small one (or just use a T-Line instead). You might have to be a little creative on where you put the pump, but they can get as small as a cigarette pack, so placement isn't much of a problem.
I like the Swiftech WC kits, but they are alot more expensive than some other kits. However, you do get what you pay for, the other cheaper kits usually perform only just as well as the best aircooling solutions, but at much steeper prices. The Swiftech ones perform much much better. They have one with a TEC, I think a 230W TEC (I'm assuming this is on 12V), which would be perfect for your needs.