well i have a 70 dart and it has 44,800 org. mi. and i was wondering if ineed to replace the valves or valve springs?and what will it do to the performance.

unless you are experiencing problems directly related to valve to valve seat seal, I see no need to "fix what ain't broke".

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'66 Cuda 225/4spd
'66 Dart GT convertible 225/auto
'64 Dart GT 340/4spd

No, unless you want to replace the valve stem seals, as they are probably crusty by now. Unless you have burnt exhaust valves (doubtfull on a low milage engine), like Bud said, there is no reason to replace them.