Hi Steve,
if the PA500 has the same functions as the PA800, he is WRONG. The Korgs are all but a workstation.

I can only comment on the PA800.

Basically it has 3 Sequencer record modes.
Step record, you type in your chord progression, put in fills variations etc. After you save it you can go to the multitrack sequencer section & event list edit any notes you may want to change, you could record additional tracks , change whatever data you want.

2nd sequencer mode is Quick Record, just press recor and you play your song in realtime. After you finish, you can do all the editing as per abover.

3rd sequencer mode is Multitrack Recording where you literally record each track individually.

The editing functions on the korg are extensive.

Sometimes it might actually be quicker to do an alteration to the actual style ie drum track may not be quite what you want , change it, save it.
Basically you could do an easy record of your song, go back and do an event list edit, to change any notes.

The are some differences between the PA500/PA800, what exactly you should be able to find on their website.
One that springs to mind is I don't think the pa500 has a sampler. It's more than likely the sequencer functions are the same. I suggest you download the manuals.

http://www.korgpa.com/pa_root/en/products/pa500_comp.html?en

and korg has a user forum that's certainly worth joining.

Been a bit more quiet lately, a lot of the regulars have sort of dissapeared into the woodwork.

http://www.korgforums.com/forum/phpBB2/viewforum.php?f=50&sid=96f6c6121ee189249dbaaf97efed7cc7

Anyway, read the manual, you don't have to read the whole thing , just the sequencer section for starters.

I don't really use the sequencer much, but if you ever want any help with style editing, I should be able to help.