Zilliak,
I personally always purchase as much RAM at the same time the computer is purchased that I think I would ever need, for example 4GB for a 32-bit computer and at least 8GB for a 64-bit computer.
That way they should be identical. Using a mixed RAM set normally doesn't work as there are so many variables, ECC, CL2, CL2.5 or CL3 PC3200 etc. And all RAM is not created equal.
lately i bought a high end workstation from a large manufacturer an it came with my extra RAM which was technically the same as they both had the same Item number. However, instead of 12GB of RAM I only ended up with 10GB, definitely a real bummer.