I had struggles getting the most out of my system when running a GTX285 and a GTX480 in the same system.
The 285 decreased it's productivity when running two WU's at once compared to the GTX480 which loved it.
In a guide on my blog i made two separate boinc work folders with separate executables to suite my needs to increase the throughput on that host.
Head over to see screenshoots and a small guide.

With this way i could set my GTX285 to use a specific executable and run only 1WU at once and the 480 ran another executable and two at once instead.

http://vyper.kafit.se

Kind regards Vyper

P.S In my example i had a 295 but most people know the drill anyhow D.S

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Addicted to SETI crunching!
Founder of GPU Users Group