Call it the "Compaq Center"(its current name) and everyone in town can give you directions.
10 E. Greenway Plaza
Houston, Texas 77046
He also perfomed at the Hofheinz Pavillion and the Astrodome in Houston, as far as I know both are still there.
Also take a trip about 90 minutes away to G. Rollie White Colliseum on the campus of Texas A&M University in Bryan/College Station, Texas. The basketball stadium was brand new(opened in 1954) when Elvis performed there March 19th and October 3rd, 1955.
While finding these few pictures I realized I have been inside four venues Elvis performed at and seen 5 or 6, without even trying. I need to go on a tour myself of venues that still remain. Sadly many, like the arena at the convention center in San Antonio, have been torn down.
Info courtesy of http://www.elvisconcerts.com/1.htm