The GTX570 arrived and has been installed in Spock.
It has run a GPUgrid work unit so far. The GTX570 completed a HIVPR_unbound work unit in 14,019 seconds compared to a GTX295 that did it in 27,728 seconds. The GTX295 can run two work units at a time. They used a different GPUgrid app (6.13/cuda 3.1 on the GTX570, 6.12/cuda 3.0 on the GTX295), so it may be more like comparing apples with oranges.
GPU temps got up to 85 degrees which was rather warm.
The GTX295 is still the best value for number crunching at this point, although that may change when a cuda 3.2 GPUgrid app becomes available.
GPU-Z Sensors tab with it under load
Installed in Spock
Another shot looking up