8800GTS SLI might have a slight performance gain over the HD 4890...But it also depends on the game you play too...
A HD 4890 is a better upgrade than the 8800GTS SLI IMO as you would have to deal with a single card, hence SLI problems are out...and would use less power, less heat...
If it is the 320mb or 640mb then the 4890 is better.
If it is the 512 then 2 of them will perform better than a 4890, but I wouldn't get them unless you already had the first 8800 GTS, the 4890 is a great card.