Compatibility of HD 7970 with a R9 280x?

[legendaswd]

Hey there guys,
Well, I'm thinking of upgrading my rig... for no particular reason at all... and was wondering if a R9 280x would be compatible with my HD 7970 (link given). I see that the two have different clock speeds as well. Will one bottleneck the other, or are they completely the same? Also which R9 280x would you recommend for my particular card?

My second question is what's the wattage needed for a crossfire between the two cards? Currently I have a Rosewill Hive 750w PSU.

Current GPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127670

Current PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182133

Thanks a lot guys!

Extra credit question: Newer cards don't seem to display their memory clock speeds, but instead show a boost clock speed... What's the difference?

 

[legendaswd]

Someone please help me!

 

[smeezekitty]

Yes you can crossfire between the two. The faster clocked card will underclock itself to the speed of the lower clocked card.
A decent quality 600w supply would be enough