I am sure they would spin in their graves! however what about using an older ati card in addition to the 5750 for physics processing? I have a crossfire compatible mobo...
They usually "keep it in the family". I.E. within 4xxx series, or 5xxx series. Though even within series they segment it.
Take for example the 3xxx series, you could pair up the onboard 3200 with a 3450, but not a 3870 + 3200. They seem to want you to mix 2 slow gpus or 2 fast gpus.