I want to crossfire two of 6670's on a Msi k9a2 cf mainboard.
As i know, when i connect 2 cards on the PCI-EX buses of k9a2 cf, they work as 8x/8x.
Will it cause a problem for 6670's? Is my mainboard supports 6670 cf technology? And i need any CF bridge?
If my mainboard is ok for 6670 CF, im thinking of buying them.
No it will not cause a problem. 6670s are generally not fast enough to saturate 8x PCI-E lanes so you won't have any noticeable performance drop over having two 16x lanes. With those cards you do not need a CF bridge, but if you happen to get two cards that have the top CF connectors then I say just go ahead and use one.
Really though rather than buying two 6670s you're better off just getting a single 6850. Of course, if you already have one 6670 then adding a second one will give you a decent increase in performance.
Definitely one 5870 assuming your PSU can handle it. Since 5870s are rather hard to find though I recommend getting a 6870 instead as it's more available, uses less power, and it should be easier to find another one latter on if you want to crossfire it.