Jack,
The 340, 540, 640, 740 and 9000 are all the current models. The previous generation was *30's, and *20's before that. The higher the first number, the more capabilities the keyboard has, and the more it costs.
You can go to Yamaha's web age and compare features. As we discussed on the PSR chat last week, the 740 would be excellent for your purposes. The 640 would be almost as good. It lacks vocal processing, but it isn't the greatest in the world on the 740 anyway.
DonM