So I have 2 major questions/concerns about this build:
1. Not so sure about the motherboard, are there any recommended LGA 1156 mobos with 2 PCIe x 16 ports (and possibly with USB 3.0 for relatively the same price)?
2. Can I run CrossFireX with different manufacturer 4850s?
A quick look at newegg reveals that this is probably it: Open Box Asus P7P55D-E Pro $142 with $7.87 shipping. This is the list of all P55 boards with more than 1 PCI-E slots and USB3 ports http://www.newegg.com/Product/ProductList.aspx?Submit=P...
Perhaps you could have a look at other e-tailers to see what else is available if you don't fancy that.
You should be able to run two different manufacturer cards in crossfire. Try and get a card that matches your first as closely as possible in terms of memory amount. Here's a thread about it: http://www.tomshardware.co.uk/forum/245454-15-crossfire...
I think you should consider something else for a CPU cooler, it might be better than the stock cooler but it's not as good as more modern coolers. The typical recommendation is the Coolermaster Hyper 212 Plus, altho according to a review on here I think that the Scythe Mugen 2 Rev.B is a slightly better option.