A DDR400 stick should work, but maybe not. Try it and see. Some motherboards are more picky than others when running different speeds of RAM.
Another more reliable way would be to visit www.crucial.com and run their memory scanner app, it will tell you what the PC should be able to run, and also link the RAM they sell for your model. They have pretty good prices also. I'd just max out the RAM to 2 gig though, 128 to 512 is a huge leap, but even 512 is low these days, and has been for the last 2,000 days as well actually :-)
If you add a re-installation of Windows to this upgrade, your PC will seem like new, better even seeing how you only had 128mb or RAM in the thing.