...The issue of USB 2.0 wouldn't have even crossed my mind; it's just so standard now that it seems like it should be a non-issue. As far as the video card goes, the 4MX is probably sufficient enough for anything I would be using the machine for. I'm a grad student in mathematics, so it would basically be a lot of online research and paper-writing.
Just to play devil's advocate:
Take your 400MHz PowerMac and bump the RAM to 1GB and the CPU to 1.2GHz with a kit from OWC. Depending on how much RAM you have now that could cost less than $300 and would run Tiger very well. You also have a ton of options for a display this way.