Right now you're not missing much without DX11-apart from a sometimes crippling drop in framerates. So unless you're having severe heating problems I'd stick with the GTX280s'.
Yea, DX11 would be an upgrade. I can control my FPS w/ the settings. As far as cooling, the cards have the accelero fan's on both. My case is an Antec 1200, so i have fans out the wazoo, and my CPU is has the corsair h50. The gtx 280's themselves are heaters, but not to bad.
My secondary pc consists of
mobo: m3n72-d am2+
cpu : x2 550 black
ram : xms2 ddr2-800
gpu : gtx 460 1gb gigabyte
psu : 750 thermaltake toughpower
monitor : 1680 x 1050
maybe i can sell one of my 280's and use that money to buy a second and swap the gpus....