Only roster spots 1-20 are counted towards the cap, 19 and 20 are "optional" spots (meaning you can fill spots 21-30 before you have to fill 19 and 20), and GA players don't count towards the cap. Any one player can only contribute up to $350,000 to the salary cap as well. Sporting Kansas City's salary spots currently look like this (as of 12/12/12 which includes the Feilhaber and Opara trades)
Which puts the total salary budget at $2,273,321. Now, we would expect Feilhaber, Opara, and Gardner to take some pay cuts (mostly Feilhaber), but we would also expect that Besler got quite a bit of a raise when he was re-signed this morning (although the pic Robb took wasn't sharp enough for me to tell ;) ). Let's just call it a net wash between those four players so far.
$2,273,321. So we still have over half a million in cap space BEFORE we pay any of it down with allocation money.