What do you want the cube case to accomplish?
Do you need Quiet?
What will go in it?
Will you be overclocking?
I am not a fan of liquid cooling except in extreme situations.
I think there are some good answers, depending on what the answers to the questions are.
Going with the Micro ATX case, you definitely get the small size and portable gaming capability.
If you're going with HTPC however you want to choose a cooler processor so the need for watercooling because unnecessary.
I recently put together a gaming machine together for a friend inside a Thermaltake LANBOX. You can get it to run quickly and quietly enough for gaming without heat being a real issue as long as the intake slits aren't blocked.
You run out of room very quickly with just the normal components, adding the water cooling components in there will just make things even more crowded. This of course presumes you want to keep the WC system internal. Its a little less troublesome if the WC system is external but then you have to drill into the case.
I really wouldn't recommend OC's in such a small case. I also wouldn't recommend water cooling in them unless you like frustrating projects. The more you want to do with the case the more it requires time, delicate finger work (it is a small space) and patience. Because those cases weren't really meant for much beyond M-atx motherboard w/ integrated graphics, low power processor, stock cooling, hard drive and an optical drive.
That said you can shove a lot of power into a small case and still not worry about heat, for the project I mentioned the specs were:
case: Thermaltake VF1000BWS Black Aluminum Front Panel
optical: SAMSUNG 20X DVD±R DVD Burner Black SATA Model SH-S203B
boot drive: Western Digital Raptor X 150GB
CPU: Intel Core 2 Duo E6850
Mem: CORSAIR XMS2 2GB (2 x 1GB) DDR2 800
MB: GIGABYTE GA-G33M-DS2R LGA 775 Intel G33 Micro ATX
Power Supply: SeaSonic 600W
Video card: GeForce 8800GTS 640MB
2 fans on it and it stays cool enough and is only off when its moving between lan parties.
The loudest thing on it is the Raptor, you can't hear the rest if you're more than 2 ft away.
Portability and watercooling aren't always ones to play nice together. However, you can do a few things that might benefit you, depending on how much you want to mod a case (minimal at most) and the amount of money you want to spend. www.frozencpu.com is a good place to start and is where I buy the majority of my components. I am in the process of fitting and configuring an additional external radiator from a transmission and oil cooler for a car...its actually a very nice looking part and has 1/2" all-copper tubing and fins. Also putting together an external radiator made from acrylic tubing.
Also, if you are wanting to cool your GPU, you would need to factor that in as well. Tubing sizes for this pump are 3/8" (I think) so all your components would need those sizes. Also, you will want tubing, and for a small case, Feser is pretty dang good. You can use hardware store tubing, but you won't get very tight bends with it.
As you can see, it isn't cheap. You have to decide if the cost is worth the DIY factor, the customization factor and the abilty to keep it somewhat 'mobile'. Otherwise, you might be better off with a decent CPU air cooler, and upgraded air GPU cooler and better case fans.
To answer geofelt, I'm looking at the Utlra Microfly for it's small size and portable gaming. I also like the fact that it can fit an ATX motherboard. The components would the best that I can buy, which I'd like help with please, being a motherboard, power supply, dual-core 3.0 Ghz CPU or better, a video card, and something that would allow to play games on an HDTV.
It looks to me like the ultra microfly case uses a micro-ATX motherboard, not a ATX. That will limit you to a single fast vga card, not sli or crossfire. That's not a bad thing with some strong cards about to launch. The vga card will be the biggest heat producer in your system, not the CPU. If the card has a good cooler, particularly if it exhausts the hot vga air directly out the back, then there should be no problem with most cases. If you have a 120mm intake fan in front, and a 120mm exhaust fan in back, then heat can be controlled. You may have to pay a price with increased noise if you need fast turning fans. Liquid cooling gets the heat away from the generating source efficiently, but the heat exchanger and it's fan will have to take up space and noise somewhere.
If your gaming needs are moderate, a 8800GTS-512(G92) and a E8400 should work fine.