As far as I know, the selection system does actually work, so let's start with the assumption that you've got something configured incorrectly. That's a good assumption to start with, if for no other reason than if it's correct the solution will be very easy to implement!
You want to run PPS LLR on the CPU and PPS Sieve on your GTX 570, correct? You should have your primegrid preferences set up as follows:
(For the purposes of this description, I'm assuming you're using the "default" location. If that's true, apply these settings to the "default" section of the prefernces. If you're using "Home", "School", or "Work", apply these settings to BOTH the location you're using AND the "default" section. It shouldn't be necessary to set both, but it won't hurt, and it might help.)
Set "Use CPU" to YES
Set "Use ATI GPU" to NO
Set "Use NVIDIA GPU" to YES
In the CPU selection column, select PPS LLR, and uncheck all the other CPU boxes. Make sure there's at least one box checked in this column!
In the CUDA column, select PPS Sieve and uncheck all the other CUDA boxes. Make sure there's at least one thing checked in this column!
Uncheck everything in the ATI column
Uncheck "Send work from any subproject".
A few things above may seem a little weird, but they have to do with some not-so-obvious behavior that has existed in the past and might exist today.
My lucky number is 75898524288+1