Can you crossfire a radeon 5770 and a radeon 4650

no

You can only CF cards of the same series. HD57xx would go with another HD57xx.

Are you planning to upgrade your 4670 with a 5770?

No you can crossfire with same cards only !

For ex : 5770 with 5770

You cannot crossfire 2 different cards !

Yes you can, but the choices are limited and crossfiring with different cards is generally not a good idea because the better card will essentially limit itself to be equal to the weaker.
http://ati.amd.com/technology/crossfire/charts.html

That link no longer works, just takes you to ATI site.

Try this one!

http://sites.amd.com/PublishingImages/Public/Graphic_Ch...