I will put it under the stairs, but I still want the noise as low as possible.
Here is my Newegg list:
http://secure.newegg.com/WishList/PublicWishDetail.aspx?WishListNumber=20924952
Still need to find a PSU.
Please comment.
unless you have a secondary hard drive, you'll most likely need more than 32GB for the ssd. to get the full benefits, you'll want to have your OS and all other programs installed on it, and only use a hdd for files you don't plan on accessing often. depending on what you're putting on it, 60-120GB would work much better.
it's always going to be a tradeoff between fan noise and cpu temperature. you can install 3rd party programs to control your cpu fan speed, and make it really quiet, but then your cpu runs hotter than it normally would, and running it 24/7 like that shortens its lifespan. OTOH, you can choose to run the fan at 100% all the time, which would be noisy, but the cpu will run cooler and last longer.
the PSU won't require a high wattage rating, since you don't need to power a graphics card. you'll want to get one with a high efficiency rating, since that's going to decide what your electric bill looks like when you're running this thing 24/7. also make sure it gets good reviews for being reliable, because if your PSU fails catastrophically for whatever reason it could fry every other component along with it.
in addition to all that, i'd also recommend getting an uninterruptible power supply if you want the system to be up continuously. otherwise if the power goes out for 2 seconds, your computer will shut off, and you'll have no way to turn it back on remotely.