I'm new to the keyboard world and want to buy a reasonably priced keyboard that will allow me to pursue the advantages of interacting with PC software, has lot's of room to grow as my skills improve and is good value - I'm looking at a PSR 620. What should I pay for a 3yr 620 and is this the keyboard for me?

The PSR-620 is a good keyboard. The sounds are real good. The PSR-620 is partly equipped with XG. The styles are also good. You can download styles from the internet. Communication with a PC (using MIDI) is not difficult.
I think that you have to pay $200 or $500 for a 3 year PSR-620.

620 is a very good starting keyboard with some professional features like DSP processor. Sound qulality is very good even styles are great and of course you can take advantage of net to find and download free styles and midi songs to use with 620. But it has hosted a DD-720KB disk drive so it might be difficult to buy 720's now a days. But you can use 2HD by a little modification (not tested).

620 'd cost you around $320 in a good condition.