P4 is your standard desktop processor, while the M is specifically designed for mobile units, while its Ghz may make it slower in raw speed terms, its architecture makes it as good as any P4. Go with the M it'll use less juice and stay nice and cool. In the case of the D505 there is a celeron M and pentium M available. If its the celeron you may be dissapointed with high resource apps.
I can get one brand new in the box for $900.00 AU (not a hot one) Is that price good?
When it retails for:$1800 AUD I think its a good deal either way.