GF4MX ain't all that
In my opinion (and experience) the Geforce 4 MX is a pretty ordinary card, not much better than a Geforce 2 MX. It's aimed at basic consumers. Even a Geforce 3 is substantially better.
If you're serious about games, and especially the amazing upcoming stuff, you need something better. A geforce 4 mx WILL end up being the bottleneck in a system like yours.
I wouldn't want to run Doom III on less than a full Geforce 4, and preferably whatever is the best at the time.
According to Carmack, the upcoming nVidia card makes all of his past work, including Doom III, obsolete. Personally, this makes me wet my pants.
And speaking of carmack comments, I wouldn't pay too much attention. Of course he ran on an ATI 9700. He's trying to sell the thing... he would have been using the best hardware he could get. BTW, out of curiosity, what sort of computer was he using? A mac or a PC? I'd be interested to know. I'd assume a PC.
Oh.. another quick point is that while RDRam is horribly expensive it IS extremely fast. DDR400 is starting to get close, and things like the nForce2 motherboard get amazing speeds from it due to completely overhauled architecture, but DDR400 is still really very immature technology, and at least on the Pentium4 RDRam is still the choice for the top end of performance.