The 6502 was released in 1975 with an advertised speed of about 1mhz but I can't find reliable information on what year faster versions became available (was 2mhz possible at launch?) or what the true top speed was. The sources I can find tend to simply list a top speed of either 3mhz or 4mhz and I'm not sure if some of them are conflating the 6502 with the later CMOS based 65c02 (circa 1983).
The MOS MCS6500 microprocessor family datasheet from May, 1976 indicates that:
"All versions of the microprocessor are available in 1 MHz and 2 MHz maximum operating frequencies."
The document is archived here: http://archive.6502.org/datasheets/mos_6500_mpu_preliminary_may_1976.pdf