One of the members of the Folding@home team, Imran Haque, has developed a memory tester for GPUs. You can find my original post on it here.
We've now moved it to its own download page and included some more text describing how to interpret the results:
If MemtestG80 detects memory errors on your GPU, we recommend taking the following steps:
- If your card generates errors and is overclocked (this includes "factory overclocked" or "superclocked" cards – anything with higher-than-reference clock speeds), reset the clock frequencies to the NVIDIA reference frequencies and see if the problem persists.
- This is especially true for the memory clock. Errors in the Logic or Random Blocks tests are likely to be at least somewhat sensitive to the shader clock as well. The upshot of this guideline is that if your overclock is generating any errors above the stock frequencies, then it's not a stable overclock.
- If after this your card still generates errors in any test OTHER than the Modulo-20 test, these errors are likely indicative of a card that's gone bad somehow. Such a card ought to be replaced.