There's a good discussion about this here:
http://www.kenrockwell.com/tech/mpmyth.htm
Ken suggests 7.2 megapixels for a 10 x 8 print so I'd say 6 would be fine. I nearly always shoot at 5 mp even though my camera offers 9.1 mp!
If you want to know why you shouldn't buy a camera with too many megapixels read this:
http://www.dpreview.com/articles/compactcamerahighiso/
If you want to understand a bit more about digital cameras try some of these:
http://www.dpreview.com/learn/?/Glossary/