Will I have hd channells if I buy a directv hdtv receiver?

My tv is not hd ,I have directv service .with a standard rca box .If i buy a hdtv receiver box will I get hd channels or better pic .this receiver I want to buy says itdecodes and has an antenna and receives in all these different signals it has hdmi connections
Update: does directv's coax from the wall deliver hd to my hd receiver and hdtv?
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