You could upgrade to 16 GB in the future but remember that requires a 64 bit operating system in order to recognize all of the 16 GB. A 32 bit OS (most versions of xp, moderate majority of vista pc's) can only work with 4 GB.
Quad core is referring to the number of actual CPU's inside of the processor. There are four of the same cpu's inside of a quad core chip, two of the same in a dual core. The major advantage to having more cores is that they can divide up the work load among the cores, resulting in faster performance when dealing with a heavy workload.
For the amount of memory in Vista, at least 4 GB! Vista with Aero effects on uses a HUGE amount of memory compated to xp when simply idling. It has been
tested and the common theme is Vista needs double the amount that xp needs. Memory is one thing that you can never have enough of, and is one of the simplest ways to boost performance in both new and old machines. Hell, memory is so cheap nowadays, it makes complete sense to go for as much memory as you can afford.
OCZ 4GB (2x2GB) DDR2800-$52 ($27 after rebate!!!!)
Good luck with the compy!
EDIT: You could get twice the system if you built the computer yourself (it's not that hard), I can throw together a configuration for you if you want.