You can definitely use that (very nice) TV with basic cable for SD viewing. And with Comcast (at least in the Denver area), even basic cable will allow you to view local channels in HD, provided whatever HDTV you buy has QAM built in to the HD tuner. I don't see that listed in the specs for the SCEPTRE so you may want to research that. Of course, if you are buying the TV primarily for xbox and PS3, this may not be of concern to you.
I have a Vizio 37" LCD and basic Comcast cable. My unit has the QAM feature so we get ABC, NBC, CBS, FOX, and WB2 in HD which is pretty nice for $13 a month.