Matthew, try using PS manages colour and then going into the printer's dialogue options and make sure printer colour management is switched off. From CS3 onwards this has been a bit of an issue and printer colour management isn't always turned off when using either PS manages or Printer manages. Wha you may be getting is both colour managing - and prints darker than those on screen is a good indicator you're double processing or double colour managing. Just wish I knew exactly where the option is to turn it off in printer dialogue.
This is why my creaking Dell desktop survives - it runs XP and CS2 and never has this problem. PS managing colour is exactly what I get
Hope that helps.
Cheers
Andy