For 200 dollars there really isn't any worthwhile upgrades from a 5850 out there. Around that price range are cards roughly equivalent to what you have ie. Radeon HD 6870 or GTX 560, the only real advantage these cards have over the 5850 is better tessellation performance, and the 6870 has better Crossfire scaling. Spending an extra 30 to 50 bucks could get you a 6950 or 560Ti, but they wouldn't be a huge boost over your 5850 for most games at 1080p. Anything above those are way above your budget. If you really want to upgrade right now, and if your PSU can support it, a second 5850 is probably your best bet within your budget.
If you can get a decent amount of cash from selling the 5850 to boost your budget, a single 6970 might be a worthwhile upgrade. Though the biggest performance boost would be in DX11 games.