IMHO; depends on the final resolution of your printing device.
IIRC; HP Color Laser printers are 600dpi... so I tend to scan and 600dpi.
Unsure of commercial ink printers.
I can tell you; the Mirror Universe PF was scanned in at 600dpi. And yes; loading that into photoshop required x64 and >6GB of memory. so I could operate on it. Takes about 4-6minutes to load into photoshop.