It have to as small as possibe and light weight, but still able to fit water cooling, would be nice if it could fit a dual radiator and mATX would be a plus!.
For a case it's self. As long as you use a single pass radiator you can fit it in the Cooler Master RC-690, they run around $80, sometimes less with rebates. If you need more cooling you could attach a 1x120 rad to the rear fan port.
I'm currently modding one to accept a 3x 120 Radiator, but it's a very extensive mod.
Problem is water cooling and small cases don't normally go hand in hand.
Agreed on the RC690. It's a nice versatile case, sweet if you can find it on one of the super MIR rebates like was going constantly a while back - ~$15-35ish shipped AR, although @ $60-80 it's still a pretty good value (but you're getting dangerously close to Lian Li prices then). And fitting a dual rad in the thing shouldn't be *that* much of a problem...
http://www.hardforum.com/showpost.php?p=1031765587&postcount=102 - A pretty nice example of a rig w/ 2x120 rad *internal*. I also recall a similar one where the guy cut a hole in the HD cage to fit the pump, which in turn allowed the bottom fan mount to be used (I believe), although it would of course limit the # of HDs you could then use... Alas, I can't seem to find the thread anymore and have long since lost the bookmark I had for it.
seems pretty great... you could mount a 240mm radiator on the top... or you could modify that core cooling system to accomodate a radiator I think... you may have some problems with the strength of the support bar and/or the weight of the radiator