If you can at all avoid it, don't rush yourself. I was there in September with a couple of old XL training pals (we were in Arizona training back in 2006) and drove from Henderson on the Boulder Highway (so as near as damn it Vegas) to the GC via the Hoover Damn (awesome, well worth a good long look) and stayed the evening in Williams - which, again, I thoroughly enjoyed.
I'd leave yourself four hours, not taking into account stop offs in places such as the Hoover Damn which you can't really just drive past. An incredible specticle.
And good advise on checking weather / taking water. You're in thin air and I was surprised by just how breathless I got scrambling around up and down vantage points off the beaten track!