To answer the 1st question, the 460 SC 1GB beats out the 5850. The review on reference 460's here speaks for itself.
2nd question, at the 1920x1080 res, you would be fine
3rd question, the case you mentioned - the CM 690 II Basic, does have good cable management and tool free installation, unless your talking about hot swappable bays. You would then be looking at cases like the CM HAF X, Corsair 800D, Zalman GS1000 Plus etc.