Not to put a downer on things, but I severely doubt that this will work. Worst case scenario you will be left with a rather expensive paper weight. There were never any X850 cards running on AGP for Mac - the X800 ran on AGP Pro but as far as I am aware no one ever got one to work in a G4 system.

Also what overclocking are you thinking of doing? You say "But, this way, after I get around to doing my over clocking, I'll have the faster RAM already!", but you won't be able to push the MDD to warrant PC2700. G4s have been stuck with a 167MHz maximum front-side bus so there isn't much you can do there overclocking wise... The bus sucks but its something you have to live with on a now legacy system like that.

I would save your money, sell your MDD and get yourself an older G5. You can pick up one of the older dual 1.8s pretty cheap these days - you can probably get one for about the value of your current system + what you are going to spend on a new card... or do what I'm doing and grabbing one of the new intel iMac 20" ers and play half life 2 all day :p