I did the L.A.-Las Vegas drive a while back, and I seem to recall signs along the highway at certain points (on steep grades leading up to the mountain passes) telling you when to turn off the A/C. Heed the signs to avoid engine overheating. The only other warning about driving is avoid L.A.-to-Vegas on Friday afternoon/evening and Vegas-to-L.A. on Sunday evening. As FTraveler mentions, tons of people from L.A. go to Vegas on the weekend, and Interstate 15 gets pretty crowded I've heard.

Yes it is HOT, but as they say, "it's a dry heat". Valet parking at the casinos is convenient and free (plus a small tip), and as long as you avoid long walks under the daytime sun you'll be fine.

Crowds are only a problem during the really big conventions like COMDEX, when rooms can be tough to get, but I don't recall any major ones in August. Check out a site like http://www.vegas.com

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Tony