>I would appreciate any information about the advantages and features that improved Visual FoxPro 6.0 over Visual FoxPro 5.0. If there are any online articles or documentation about it, it would be very helpful for me in order to encourage a customer to upgrade.

Hilmar told you about Advantages, but you should be thinking about VFP8 Upgrade :-)
At UT-Magazine you could find some of VFP8 Advantages.