I can only get them to run in Dual channel mode at DDR266, everything above that powers the machine down!
Haven't even thought about trying DDR500 as I can't even get it to run at ddr400.
What I did manage though was getting the thing to run DDR400 but I had to drop the voltage to 2.5V(manufacturer rec. 2.8v) But that ended up being ropey after playing battlefield 2 for a while.
I tried my friends ballistix (non-tracer) in dual channel and they worked fine with the same recommended settings as for the tracer albeit the difference was mine are 2x1GB and his are 2x512MB.