I'm trying to use an Nvidia GT520 as a render device, but i have an AMD HD7770 as my main GPU. In user preferences i can choose CUDA as my compute device, and it detects my card, but when it's time to render, cpu usage is at 100% and it uses my main system memory. My monitors are connected to my AMD card. I'm using Windows 8 64 Bit, with both latest AMD, and Nvidia drivers, and rendering on Blender 2.72
Silly me, i completely forgot about the option in the renders tab, i thought the user settings was enough. Anyway it does appear to work, fur mark does show the GPU heating up.
It isn't very useful due to it's low memory and little processing power, but being on a separate GPU i can render lesser projects while doing other tasks.
I did NOT have to enable Experimental Feature Set. do not know if it changes anything.