Please read performance reports for listed hardware in this section
http://forum.1cpublishing.eu/forumdisplay.php?f=191
CloD needs a 4 core CPU @3.5+ (I think Intel ones are faster now).
AND
A high-end video card to run maxed out (like GTX580 or at least HD6970-2GB). ArmA-3 will also need it next summer.
It's up to you to decide what to upgrade first. I decided to start with CPU. Will get a videocard in 6-9 months when the nextgen cards are out.