Additional monitors eat up very little ram so the 5850 can do eyefinity with three monitors with ease (some games might chug once you go there
though ). You can occasionally get weird issues with clocks not downclocking or downclocking too much when more monitors are hooked up but as far as power it's more than fine. Not sure why no one didn't answer this sooner.
As far as a new GPU goes as long as you're happy with the performance in games then that's what really matters in the long run. Though I would advise maybe a cheap 7850/7870 ( or 660 for an NV card) for a nice upgrade in performance and more vram if you go for a 2GB version. Though it's not really needed it would be a good boost all around for gaming.
Streaming will still be mostly affected by CPU and upload speed so don't worry too much about the gpu from that angle. Edited by Junkboy - 7/25/13 at 7:19pm