Dual Cheap GPUs vs. Single More-Expensive GPU

I'm in the process of building a computer, and I'm stuck on the graphics card. Where I live (Aus) I can get Radeon 6450's (1GB) for AU$39 each, and I'm wondering if getting 2 of these and running them in CrossFireX will be better than getting a single HD 7770 (1GB) for AU$109. I'm looking at running 3 1280x1024 monitors, I have 3 VGA and 2 DVI ones, and I can use any combination of them, whichever works, with any adapters necessary. I decided to ask here because the websites I've been looking at all say different things about CrossFireX and Eyefinity compatability.

I'm trying to keep the price of this build low, as I am basing this around an Intel Core i3 2120 @ 3.30Ghz (1155) which I got from an old Dell computer, and a fairly large case from another old Dell. The processor works fine, and I'm using it in my current machine, but it has integrated graphics and I can't any games.

My second question is about motherboards. As long as it supports the graphics cards, the processor and has one SATA 6Gbps port I will be fine with it. If possible, another preference is that it has 4 RAM slots, because I have a bunch of 2GB sticks that I am currently using, and it would be cheaper to use these instead of getting new 4GB ones, as I want 8GB minumum for HD video rendering/editing.

Any help is much appreciated, thanks in advance. 

If you can get the single card then its better.. especially since the beta drivers offer up more performance now. As for the motherboard.. I'd look to see if your psu for your dell computer is standard. I haven't looked into it recently but from what i remember dell used to use their own proprietary 20-24 pin layout. Might damage the newer mobo.  As for motherboard... anything is fine..can't really tell you much without a budget range to work with. I have seen complaints with gigabyte and their marvel control for sata 3.0 so I'd be careful and just stick with the native intel sata. 

I was going to get a new PSU for this build as the current one is only about 240W maximum. What do you think of the MSI B75MA-P45 motherboard? It's about AU$61 http://www.msi.com/product/mb/B75MA-P45.html#/?div=Detail

I would be willing to pay up to about $150 for a motherboard, as I want something quality and the ability to easily upgrade components as I go.

Thanks for the advice.