So if i bought a plasma and it was 720p/1080i

it would have a native resolution of 720p, but if i hooked a computer up to it could I display the image at 1920X1080?

And another question regarding HDMI and DVI,

I read somewhere that HDMI can only broadcast 720p while DVE can match a screen resolution of 768 which is what most are coming in. Am i understanding this clearly?

it may accept different resolutions but it will only display it in it's native resolution, so no a 768p plasma will not be displaying 1920x1080i/p.
you can transmit dfferent resolutions over hdmi, it is not restricted to 720p.

some sets or sources may be limited as to what they accept/display or output over their various inputs/output but that is not limited by the hdmi itself it is from the sets or output sources manufacturers.