> What's this 4Mhz vs 20MHz?

IIRC (as in I haven't checked the datasheet) the 877A version always comes
in 20MHz, where the non-A version does have two speed versions. Others have
already pointed out the 44 pin and other differences.

All the info about these variations is at the end of the data sheet where it
gives the ordering codes, and points you at the package differences.

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist