I believe it will work but have never tried it. I have 4GB (4x1GB) of the DDR2 800 Ballistix Tracers in my GA-p35-ds3r. The voltage, timings and speed are the same so it should work. I've worked on machines before that have had mixed brands of ram with no problems. As long as the voltage is the same the machine usualy defaults to the slowest speed ram.
I guess the GA-p35c-ds3r has 4 slots for DDR2? I know it suports both DDR2 and DDR3 just not at the same time.