I have the SE13HD equivalent of your projector. You'll need an HD receiver, not just any old cable box. There are some rare cable systems that put out an HD signal. But just because it says "digital cable" doesn't mean your cable system carries any HD content.
In any case, assuming your source is cable of HD output, it would be either via the Y,Pr
,Pb or the RGBHV inputs. On my projector I've used both the RCA DTC100 (RGBHV) and the Zenith 1080 (YPrPb) both at 1080I.
Even with one of these receivers attached, only certain content is actually in HD. There are several local off-air sources here, as well as 2 HD channels on HBO via satellite. You'll have to get specific information on what HD content is available in your area. As I said it's extremely rare yet to be able to get HD via cable. TWC has it in some locations, and Comcast has announced it in a couple of locations.