Yes, you can use AVS with 400mhz.
It might be a bit slower and youmight have to run them in windowed mode, but they'll run.
To find out how to use AVS, scroll up to the top of this screen and click on the tab titled "Develop".
Note that on the winamp site, AVS is called WVS (Winamp Visualisation Studio),
Click on the tab marked WVS.
Read the help and "how to use" files.
Then have fun.