My mushkin redline can do 270 @2-2-2-6 rock solid, but out of curiosity I put the slower stick on dimm 2 (further away from cpu) and put the faster stick next to the cpu....it completely wreck havok the memtest, got thousands of error. switching them back eliminated the error and gain back the solid memclock I had.

If I were to run 4x512MB I definitely would put the slowest one next to cpu and the fastest on the last slot