I bought two of these yesterday - one of them was the updated AGP 8x, the other was the regular AGP 4x. You can tell the cards apart pretty easily: The 8x has a HUGE copper heatsink with embedded fan which covers both sides of the card around the GPU. The 4x uses a small blue orb with embedded fan.
I was quite surprised to find the 8x, when both boxes clearly listed they were AGP 4x. The only appreciable difference I could find between the two, is that the shrink wrapping on the 8x was the more pliable kind accented with air holes at regular intervals - the 4x came in the more rigid, less flexible shrink wrapping. Also, the plastic insert inside the box that contained the 8x was about 4" shorter in length than the 4x - if you notice the innards of your box shifting around considerably, it's likely you scored an 8x.
That's about all the advice I can give you
For the record, the 8x is clocked at 250/500. I haven't checked the clock speed of the 4x.
-Cris