You can crossfire on your mobo (crossfire/SLI ready tag is just a marketing gimmick).
The only problem that you will encounter was best described by serge44
'2. I would get a MB that can at least run them on 8x8.. 16x4 will definitely limit the bandwith for the one running on 4x, while 8x wouldn't be a problem. It's not that anything bad is going to happen, it's just that you wouldn't be getting all the juice out of your second card as you should. 'http://www.tomshardware.com/forum/295285-30-m4a87td-crossfire-questions
(link to the above article)
Also the difference in your mobo, with it's 16x and 4x PCI-E, and a mobo with 2 16x slots is demonstrated here:http://www.tomshardware.com/reviews/pci-express-scaling-p67-chipset-gaming-performance,2887-10.html
I hope that answers your question.