I am having this exact same problem, using a Landsat Thematic Mapper (TM) image of the Imperial Valley, CA, region. I create a training site, fill out the training sample manager icon, and when I hit the histogram icon, I get an "Out of Memory" message. I see that the service pack was supposed to fix a histogram problem with Landsat images, but I have installed the SP4 already.
Thanks for any advice on this issue!