The performance of GTX275 is supposed to between GTX260 and GTX285. There're rumors indicating that GTX275 is an overclocked edition of GTX260, which is untrue.
According to our source, GTX275 features 240 stream processors just as GTX285. Equipped with GDDR3 memory to form 896MB/448bit, GTX275 is coming with core/shader/memory clock of 633/1404/1161MHz.
Considering the default frequencies of 648/1476/1242MHz and memory interface of 512bit found on GTX285, their performance should be very close.
NVIDIA aims to beat HD4890 with GeForce GTX275, but GTX275 will naturally affect the selling of GTX285, so this would be a problem for NVIDIA.