not to sure how to answer question one, my advise is to learn how to do it yourself that way if you have a problem , you may be able to pinpoint it easier. It is not hard at all and the zen was one of the easest build i have ever done, including the HDD mod and fan change.
2. anything from a 2.4C to 3.2C is great. i would not get a prescott because they are really hot.
I have this ram in my zen right now and it is great
http://www.newegg.com/app/ViewProductDe ... 026&depa=1
Corsair, OCZ, Kingston, Crucial are all great companys
I you really like to use alot of apps at once then you may want to get a gig of RAM. i only use 512 in the zen and it works great still.