Hello everyone, just wanted to give an update - especially for all of you who recommended an upgrade of the whole system...
I managed to get the system running at almost 30fps on medium settings, 1680x1050 resolution, and shadows turned off. Here's the quick story:
Bought some new RAM, plugged it in and it didn't work (I think it's a high density / low density thing). When I replaced old RAM to make sure that I didn't break anything, I place the mis-matched RAM into the system in the proper order (someone else had bought the upgrades long ago). That is, the 1 gig sticks went in the primary slots and the 512 sticks went into the secondary. That's it!
Thanks for all your help - and add this to your knowledge to help other people - this system configuration will work!