You certainly have enough power in that Box.  I'm running Windows 7 64Bit Ultimate on an AMD Phenom II X4 955  BB Edition, 8GB Corsair RAM, XFX ATI Radeon HD 4870X2 2GB Video wtc, and like I said, I have had an issue which is just about identical to what you described.  One swapped DIMM fixed it.  Carrara isn't as forgiving about memory errors as some other packages.  I guess it's one of the prices we pay so we don't have to spend thousands on stuff like Maya.