One of the members of the Folding@home team, Imran Haque, has developed a memory tester for GPUs. You can get a copy of it here:
MemtestG80 is a software-based tester to test for "soft errors" in GPU memory or logic for NVIDIA CUDA-enabled GPUs. It uses a variety of proven test patterns (some custom and some based on Memtest86) to verify the correct operation of GPU memory and logic. It is a useful tool to ensure that given GPUs do not produce "silent errors" which may corrupt the results of a computation without triggering an overt error.