If your main purpose is gaming and you are fine with buying a refurbished product, the the G71 from buy.com is the much better choice. It has a stronger CPU, a much better GPU and comes with more RAM.
The only two cons for the Asus G71 is that it has a low screen resolution for a 17'' unit and it's a refurb, meaning that it was return for any number of reasons, fixed if needed and put back out for sale: http://www.buy.com/retail/reconditioned.htm
The last one is a little more expensive but it has a better processor. Perhaps the price will go down for Thanksgiving/Xmas?
I've read that the G71GX can get quite hot and the battery life is poor...which makes me lean towards the HP a little bit more. Plus, like I looked up, the HP would have a longer and cheaper warranty through SquareTrade.
The Quad is only worth it if your programs can take advantage of more than 2 cores. Since the quad has a lower clock speed, it will actually perform worse in single and dual threaded applications (applications that only use 1 or 2 CPU cores). So I guess my vote is no, it's not worth the extra $150.