There are a lot of CorssfireX motherboards in the under $150 region with PCI-E slots that run at X8/X8 mode.
Here are some Motherboards that I would recommend (I looked into some of these while I was "doing my homework" for my new rig):
890GX motherboards (these have on-board graphics available)
- GIGABYTE GA-890GPA-UD3H Costs $134.99 (price of it hasn't changed at all in the pst 3 months)
Supports the new Phenom II Quad and six cores, 2 PCI-E X16 slots (x8/x8 when in Crossfire), SATA 6/Gbs, USB 3.0.
http://www.newegg.com/Product/Product.a ... -_-Product- ASUS M4A89GTD PRO USB3 (there is one that doesn't have USB3.0m while another one does) $137.99
Again supports Phenom II's, 2 PCI-E slots (x8/x8), SATA 6Gb/s, USB 3.0
http://www.newegg.com/Product/Product.a ... -_-ProductMSI 890GXM-G65. $119.99 after rebate. USB 3.0, SATA 6Gb/s, x8/x8. It just seems that MSI boards are either a hit or miss; DOA or works great.
http://www.newegg.com/Product/Product.a ... -_-Product890FX boards are typically better than the GX, they have no on-board graphics, have more 2 PCI-E X16 slots or more, two or more cards can run at x16/x16 instead of x8/x8, and seem to OC better. But they are more expensive $140 - $200+
(need to split my post into two)