Honestly I would keep your current card. Unless you are looking at upgrading to its new gen card, the 5850, I really wouldn't bother. Usually you want at least a 50% increase to really vouch for an upgrade. The 5850 would actually give you closer to a 100% increase. DX11 is nice, but not a reason to upgrade.
Like said above a 4850 performs just under a 5750.