For PCIe bandwidth, 8x/8x is enough for 5850s. You really only need 16/16 for, say, GTX 580 or 6970 cards (or 590, 6990).
But anyway, all P55, P67, P68 boards only have 16 PCIe lanes to work with. The only way you get 16/16 on one of these boards is if they use the NF200 chip which works great, but it's only on the most expensive boards.
That's kind of why X58 is still a good choice. Sure the CPUs aren't as good as a 2600 but you get a better motherboard (32 PCIe lanes). Otherwise gotta wait till Ivy Bridge which should be the X58 equivalent Sandy Bridge