Yup, only difference is the clock speeds. Which technically speaking you can do yourself, and in theory save money. But for some, they pay more for higher speeds that are guaranteed. I for one, spent less on a Foxconn 8800GTX OC, and got higher clocks than other brands 8800GTX, such as eVGA, and their clock speeds were lower. So it really varies some of the time, really depends on the manufacturer and how much their willing to clock and than sell at.
If you're going NVIDIA though, I highly recommend Foxconn. The 8800GTX OC from them will be my second product from them, currently I'm using a 7900GS OC. Plus more often than not they come with rebates, and are generally cheaper anyhow. But, brands liek eVGA offer a step-up program, and have the proven reliability and customer service that so many people have come to know. Basically, this is one of those, "pick your poison' scenarios...no matter what card you end up getting you'll get reliability and a great card. So feel confident within that aspect.