2.5 sounds even to me: 2x 256MB + 2x 1024MB. I booted up my old box to verify; it's actually got 2.5GB in I'm pretty sure exactly that config. This box has 3x1GB alongside 3x4GB (I think I got carried away.). They should just be using the best common timings and frequencies, provided they can operate in the same range and they are the same rough type. Though I do have the memory slots in corresponding channels using matching mem sticks, so I couldn't say if you could get away with completely mixing it up, with like 4 slots (double-channel) being: 1x256MB, 1x512MB, 1x1GB and maybe an empty slot just to screw with it.