An Athlon 900 is plenty sufficient to run any of these tables, even attack from mars and indianapolis 500.
1. Make sure you're desktop display depth is 16-bit high color (not 24 or 32 bit true color)
2. Make sure only one table is open. It is possible to have multiple tables open in Visual Pinball.. close them all and make sure the only open one is the one you're about to play.
3. Make sure nothing CPU-hoggish is running in the background.. findfast, application-start tuneup, browsers with java/flash/etc displaying, etc.
Try these and report back :)