What offerings do the XX line of cameras offer over the Rebel line when it comes to flash photography and options(pertaining to flash)? PC sync port, sync speed, what all?

The XXD line has a PC socket and 1/250 X-sync speed.
Starting with the 40D and the 580EX II, there is additional flash control via camera menu (vs. making adjustments on the flash itself)
That's about it. The E-TTL system is otherwise the same.

Good deal, thanks for summing it up for me.

I just thought there may of been more custom function settings.

Oops, forgot one.
With the XXD cameras, there is a custom function to make the flash not fire if you just want the AF assist light.

Using the original rebel (300d) there are no real flash options so no 2nd curtain sync... at least with official firmware. The only flash related setting is red eye reduction