custom 404 & search engines

[kaibobble]

Does anyone know how search engines deal with custom 404 error pages.

so for example if you set a site map as your 404 page would search engines such as google carry on following links or would it recognise it's a 404 and stop or rank you lower in the search engines for it?

[rhsunderground]

i think this might be better suited for the search engines forum, perhaps?