Here is my situation: right now I have 512MB of PC3200 RAM and my FSB runs smoothly at 200 MHz. I have a 256MB stick of PC2100 RAM sitting around at home. Would it be beneficial to put that in and have 768MB of RAM, but a FSB speed of only 133MHz?
No not even close dude. That's a huge bandwidth decrease for a minimal amount of RAM increase. It's like shooting yourself in the foot and saying "well I have less body mass so I can run faster right?"
Especially with a 64bit system you want nothing less than pc3200 in there