What? Folding relies of FLOPS, and thats an area where ATI cards have an edge. The 5770 has something like 1.08 TFLOPS single precision performance, while the 260 has something like 600 GFLOPS single presision.
PS: The reason for this is because while Nvidias 200 arch has higher clock rates per core, the 5770 has much more cores, resulting in much more parallel processing power which is what folding needs.
no, the reason ATI is so bad at folding is the outdated GPU core for ATI cards (been the same from HD3xxx) and can only utilize something like 320 stream processors (instead of 1800), even something like the 9800GT will beat a 4870
EDIT: 5770 Single Precision Floating Point Flops: 1360 GFLOPS, GTX260 Single Precision Floating Point Flops: 874.8 GFLOPS, and be happy that F@H doesn't use Double Precision which the 5770 can't do at all