I use a XFX 650i Ultra motherboard and I cannot use both ram modules together. I have tried "underclocking" both modules however Windows Vista will not boot up (blue screen of death). However if I use one or the other it works perfectly.
To be more specific:
OCZ 2GB PC2-6400 EL Platinum XTC Edition Rev. 2 Dual Channel DDR2 Kit (2 x 1GB)
To see if the problem is incompatibility between the two module types, try installing 1 module of the OCZ and 1 of the Corsair and see what happens. If that works fine, your issue is likely the 2nd mentioned by Yb.