I'm sure there will be AM3+ boards that support SLI, just like your AM3 board does but crossfire will be much more common. Supposedly Nvidia charges $5 per motherboard for SLI certification while Crossfire is free. This is why every dual slot board(except ironically those with an Nvidia chipset like yours) supports Crossfire while SLI is more rare.
As for whether or not you should get the HD5850... it's up to you I guess. An Nvidia card like the GTX 560 Ti will give you the option of SLI in the future. Like I said earlier it is about 20% faster but costs about $100(or 71%) more. So you are paying quite a premium for the extra performance + SLI option. Whether or not it is worth it is up to you.