I think its most likely that faster gpu's need more cpu usage, yes.
Im getting full usage on both cores, but look how much ppd and how much faster im folding.
I guess if it works for you, then continue to fold with both smp and gpu.
My other box will be up not too long from now, i estimate within a week or so and ill post ppd results from that one, also folding with an 8800gt.