Can anyone tell me what order in strength the different gradings are. e.g. 8.8, 10.x, 12.9, A2-70 e.t.c.

I know the "numbered" ones get stronger as the numbers get higher. I think A2 etc might be stainless bolts.

how do stainless grades compare to steel grades

All in newton mm squared
steel 8.8 Tensile strength - 800, yield strength 640,
steel 10.9 TS - 1040, YS - 940,
Stainless A2-70, TS - 700, YS 450,
Stainless A4-80, TS - 800, YS - 600
Straight off the top of my head actually out of kit car mag aug 07

Stainless is strong but can suffer from stress corrosion cracking in hot stressed environments.

<makes note to self to replace stainless threaded rod in engine mounts>

jk

http://www.americanfastener.com/technical/grade_markings_steel.asp