Accoring to other documents, PCI-E x1 has 250 MB/s transfer rate in one direction (not 256MB/s). Because PCI-E has 2.5Gt/s, and using 8B/10B coding, the bandwidth should be 2.5Gt/10 = 250MB/s. (1G = 1x10^9 when calculate frequency)
The data listed below probably has some errors.
http://www.tomshardware.com/motherboard/20040619/socket_775-28.html
The data listed below probably has some errors.
http://www.tomshardware.com/motherboard/20040619/socket_775-28.html