How much, successfully? +0 is all you're guaranteed. "Safely".... You can't really do any damage to the card, without modifications to the power draw etc. Increase you core (or mem) clock slightly, stress test, rinse & repeat.
Typically, the 5500XT is more power restricted than anything else. There are soft powerplay tables that can allow 5500XTs to reach up to 2100MHz. You're not going to achieve anywhere close at stock power draw... but there should be some additional. How much or how little though, is anybody's guess.