Well at least you sort of have an idea what you want, I can tell you now that crossfire is epically better and its incredibly smooth.
Having installed 2x 280X recently I can tell you now AMD have been working their ass off with drivers and stuttering and I can clearly see the difference.
I am personally a fan of running 1 powerful card but right now the 290x isn't available locally and won't be for another 3 weeks or so...
If you can't wait I would say try getting 2x R9 280X Graphics cards.
Gigabyte R9 280X Windforce 3 edition, is dead silent while providing ample cooling, really good card, comes with a nice overclock out the box as well.
http://www.rebeltech.co.za/index.php...ducts_id=11135
You could also look at the
Club3D R9 280X RoyalKing Edition. Healthy oc out of the box, comes with a sexy black pcb and backplate and cooler should be pretty decent.
http://www.rebeltech.co.za/index.php...ducts_id=11129
For the time being I would avoid the 770 and 780 to see how pricing drops.
As for Mantle, we don't really know all that much about performance but from what I understand and how EA are raving about it, it might just be really amazing. I would say around 20% extra performance but that is a guestimate from my side.