Looks like you have the 470 or 475. There is also an opportunity to upgrade the processor (and get some nice performance gains)!
Take a look at
this site and
this site. You will find the info provided to be useful. In the first site, the 470/475 forum has a sticky about upgrading the RAM.
If you search in both of them, you'll find info in upgrading your MSS and the folks that work the forums are very helpful.
I got a new 485 in Feb and added 3 1 TB drives. I am in the process of moving my DVDs onto the server. I also got a Popcorn Hour A-110 so I could stream the DVDs (using a wired network) to the home theater. I added YAMJ and the 5150 skin (better GUI for presenting the DVDs on the TV). Very cool.
I mention the Popcorn Hour to help you with the opportunity to spend more money! If you do some research, you are gonna HAVE TO get one!