This part of the build is a no-brainer, DDR2 memory modules are much cheaper that what they were about 3-4 years ago. I currently have six sticks of 1GB Crucial Ballistix but that might be too much of an overkill so I’ll probably just sell those. I recently purchased a 4Gb kit of Team Xtreem Dark (2 x 2Gb modules) for $85 and they seem to be running just fine or I could go even cheaper and grab the Kingston 4Gb kit for $63.
*Prices are obtained from MSY Website
I’ve yet to decided which Operating System I will use for this HTPC but I think the 4GB setup would be the sweet spot.