I am also having this issue and has been troublesome for months. Just in the past couple of weeks the problem has developed a rather irritating and fascinating twist. After the message appears "# textures discarded due to insufficient memory 1 or 2 or 4 or 6" I either crash or get a flashing texture show on some items in my viewing range. Random textures flash on mostly larger thinner prims. I have tried all the above suggestions with no results.
I use a iMac Processor Name:Intel Core i5 Processor Speed:2.66 GHz Number Of Processors:1 Total Number Of Cores: L2 Cache (per core):256 KB L3 Cache:8 MB ,Memory:8 GB
graphics::ATI Radeon HD 4850
Come on Lindens or someone.. There must be some sort of fix for this! Please?