I'm currently in the process of building an mATX system for a friend's daughter. She definitely wants one of those Aspire X-Qpack2 cases, the cubed shaped ones. It'll be used mainly for WoW, homework and internet browsing. Maybe some Portal or HL2. I'll be replacing the PS that comes with it with a Seasonic Modular PS seen here, and possibly replacing the fans inside of the case with a more reliable brand.
I'm concerned with air flow in this case, so my primary question regarding the choice of video card is: Would a shroud-type reference card, that pushes hot air out the back of the card be a better choice than something like an egg-shaped or vapor-x cooling fan/heatsink that pushes hot air into the case? I will probably replace the thermal compound on the GPU with some OCZ freeze to help keep temperature lower in case they decide to ever OC the GPU.
Extra Credit: Is it feasible to zip-tie two 120mm fans together to better exhaust the heat from the case?
I'd stick with the stock cooler, even under heavy loads it's quiet and although the HD5770 is not exactly a room heater it will not hurt to eject its heat out of the case directly.
Replacing the thermal paste will invalidate the warranty, unless you're thinking of heavy overclocking where every degree makes a difference, I'd leave it alone.
Extra Credit: Yes, or you could use a suitable glue. However, a single high efficiency fan will give similar results with less noise.