Pretty much any mobile Radeon will spank the 855GM - I've heard 3DMark 2001 scores of about 1500 quoted, compared to 6000 or so for a Radeon 9000. (Just as a rough indicator, that is, before the 3DMark police flame me)
The 855GM uses shared memory and I'm fairly sure it doesnt have hardware T&L, so it's only a DX7 part. It'll play older games well enough but for anything recent it's well worth paying the extra if a radeon is available.