Hello txt.file!
The GPUgrid project forum is kinda jammmed with information (in a random fashion it seems to me). One of the project 'issues' that trip people up, is the 'deadline' for wu completions (4 days). A 'marginal' GPU might complete the wu on time if run 24/7. Some 'marginal' GPU's will not...
Right now I still have a 8800GS (96 shaders), that takes ~23 hours to complete a wu. While this is an 'acceptable' GPU, it is not actually one of the slower GPU's. This GPU could still complete a wu on time, even if it only crunched for ~8 hours a day.
I posted a link on the 1st page, that should take you to a page (on GPUgrid) that gives a good listing of GPU's that will work on this project. 8800 series cards do quite well (for GPUgrid and Folding@H).