GPU Upgrade Conundrum

I'm a background reader, but I could definitely use some help!

Current Rig

CPU is sporting OCZ Vendetta 2 Triple Heatpipe.

All packed into a Antec Three Hundred Mini Tower Gaming Case.

ATI is what I'm familiar with, but NIVIDIA is fine aswell

What I've been thinking is acquiring a second 4870 and run the two via crossfire. This presents its own issue as my current motherboard only has a single PCI Express 2.0 x16 (I was not thinking ahead ), not to mention the fact that my beloved X-FI Titanium FATAL1TY would likely be in the way.

So my real issue is choosing between a new GPU to replace my current 4870 allowing me to keep my motherboard, or purchasing another 4870 as well as a new motherboard. Either choice would present itself with a component wasting away in a box until needed.

To summarize, my quesiton is what would be the more logical choice in the sense of price and power. A second 4870 and new motherboard, or a level of GPU all together?

Note: I am a Canadian resident so please no links to american sites
Preferred Sites: Direct Canada, New Egg or NCIX

Much appreciated,

-Instance

Excellent!

This is indeed what I was personally leaning towards.

My e8400 is currently sitting at a comfortable 3.6, which I've read shouldn't have any obvious bottlenecking with dual 4870s. I still have much room (temp wise) to OC further.

I very much appreciate your help Niklas, especially the links.

Cheers,

-Instance