Depends what you want to use them for. Most dedicated film scanners are (or were) 2500dpi and up. I think the last Nikon Coolscan was 4000dpi.
The thing is you can always throw away resolution after you have scanned, for instance a re size to attach to an e mail, but if you don't have it in the first place you are a bit stuck if you want to do anything other than look at them on a computer screen.
If you are printing you will need at least 150dpi at the print size but preferably 250-300 dpi. A 600dpi scan on 35 mm will only give you 900ish pixels on the long side - a 3 inch wide print??
Also it's surprising what detail you can pick out in a high dpi scan that was often not visible in a 6x4 print.
So I would use the highest un interpolated resolution the scanner will muster, and be prepared for some large files.
HTH
David