Alas, I recall major troubles with the GF2 MX400s, they were a particularly weak card that was not able to run BoB2 at release.
However, a possible suggestion, I note that there are some fairly cheap cards on offer at ebay - I checked for AGP cards in nVidia 6800, and 7800 AGP, the former run $20 (just one!) and up used, 55 and up new, while the 7800s run 36 (again, just one) and up used, more typically >100, and the new ones are too expensive to consider. A 6800 would be a workable card and a pretty good match for your system, without being too power hungry (no point in a card that will need a new power supply), but I suspect that the 512MB ones they offer will want too much power, stick to 128 or 256MB. Here's a link to the cheap one currently up.
I've broken the url in half as otherwise it makes a complete hash of the forum page. Just paste it back to one line to use it.
This type of card won't make your system able to run BoB2 with anything like full features, but it should be able to manage at a low level; it will certainly run BoB1 beautifully.
