It's the speeds of the PCIe 2.0 slots. Normal speed is 16x, but when you add a second card, it might decrease. The board you picked is 8x/8x. It's generally not worth the extra $50-100 to get a true 16x/16x board.
Seeing as you haven't bought parts yet, you might want to post what you're looking at. I'm sure we can give you some better parts/prices. One thing I can say for sure is that you should get the X4 955 instead of the 965. The 965 is just a factory overclocked 955, so there isn't a real reason to pay more for it. Also, you should consider some 7xx chipset boards (the Gigabyte GA-790XTA-UD4 is excellent). 8xx chipsets cost more, but don't really give you much for the cost that you wouldn't get with any AM3 board. For example, the big draw is the ability to drop in an X6 CPU (which you won't need) without a BIOS update (which is free).