The Environmental field is very competitive so I was always told a Masters would be a good idea, as long as it's quite specific (e.g. an MSc in Environmental Impact Assessment, or Wildlife Conservation) and not a generic 'Environmental Science' Masters. I think they are more like conversion courses for people coming from different areas, though I'm not too sure.
Depends what field you want to go into though. If you wanted to work in an Environmental Consultancy then I get the impression from when I was applying for jobs that a Masters and a year or two of work experience is pretty crucial.
Hope that helps.