If you plan to buy yourself 2GB of Kingston HyperX RAM (KHX8500D2K2/2GN to be precise) for a Gigabyte GA-P35C-DS3R (BIOS Rev. mainboard, beware: I couldn’t get it to work. I tried:
- DDR2-1066/PC-8500 and DDR2-800/PC-6400 settings
- 1.8V and 2.2V
- Installing both modules in bank 1 and 2 (instead of 1 and 3)
With DDR2-800/PC-6400 settings and 1.8V, the RAM would pass MemTest+ V1.70 but some applications would crash reliably (comix, for example, when opening the third file).
I’ve now replaced the modules with 2*2GB from G.Skill and the board is rock solid.
Dark Views 
I have the P35C-DS3R and the 2gb kingston memory (KHX8500D2K2/2GN). Iam getting Blue screens of death while playing games. Can this be caused by incompatibility of the two components like you said?
Try to run Memtest86+ (http://www.memtest.org/) and/or replace the memory.
Note that Memtest86+ can’t find all problems; so even when all tests are green, you should still try to replace the RAM if the problems persist.
Note 2: Replacing DDR2-1066 with DDR2-800 won’t reduce the system’s speed as much as you’d notice it.
I ran memtest86 for 9 hours, but no errors were found, but I replaced my Kingston with a GeiL DDR2-800 2GB kit and everything works fine now. So I think that the Kingston memory I had, just isnt compatible with this motherboard.