I have a side bar utility on my Vista box and it monitors amount of Ram used. I never see it go much above 900mb and runs mostly in the 600-800mb range. 2-4Gb should be more than enough for a HTPC.
Water coolers are nice but there is still a fan that can make noise just like regular CPU coolers, it is just the fan/radiator is now remotely located. The water coolers are more effective for overclocking.
I went with a fanless power supply to help keep the noise down on one of my recent builds. The other build had a large 120mm fan on the P/S, both boxes are running the same cpu cooler, I think they make the same amount of noise, about as much as a laptop when the fans are kicked in.
