If you must have a modular PSU, the Corsair HX520 is about all I'd recommend.
Case wise, Antec 300, 900 or Coolermaster CM690. All decent mid-towers that are affordable.
And the HD4850 you get, make sure it's non-reference cooling so it'll be cooler and hopefully quieter. I quite like my HIS HD4850 ICEQ4 Turbo, nice dual-slot cooling which works well. OCs pretty well too
