I walked these up from a stock 250 to 277 over the course of an hour and a half while running SP04 Large FFT. Then I thought, this isn't stressing them enough... 277 is pretty high for these sticks already. I don't know why Large FFT is considered a RAM stress when its footprint is generally very small. I never saw the SP04 processes eating anymore than 20MB of RAM a piece. In contrast to running a Blend test which will occupy most all of the RAM it can get.
So I went to a 32M(dual) SPI run... no problem (~256MB per instance). Then onto some 3DMark 01, 03, & 05 runs... 3D is what I've read these sticks have problems with. Many reports of 2D stability up into the high 270s, but only 3D stability into the mid 260s.
I'm gonna let em' fold for the next day or so... I've got some large WUs on that rig right now and they're very good at detecting memory stability. If two instances of this large WU doesn't bomb, then I'm pretty confident dual Blend tests would run as well. I'll probably do some Blend first thing next week as I want this rig available for weekend gaming... which will also help me tell if the dimms are stable.