For the purpose of mining, don't really bother with having as many 16x slots as you can. Instead, go for max amount of PCI-E slots, or 4 max if you intend on using dual GPU cards such as 5970/6990/7990, and using 1x > 1x pci-e ribbon cables since 16x > 16x dont bend very well and can break. I had 4 16x > 16x and they all eventually broke (places where the pins solder on broke). Now I use 1x > 1x and my cards are happily mining away. I had to short presence pins on all slots that I had 1x > 1x ribbon cables in but it's fine with me.
Right now there is a 6-slot board on newegg for $110. http://www.newegg.com/Product/Product.aspx?Item=N82E16813130565R
BTW if you DO decide to use 1x > 1x cables, you'll very likely need to file off the end of the slot to allow plugging in a 16x. Had an extra 1x > 1x lying around still in the bag to show a before/after.