Memory won't increase, it will mirror the data so that both cards can access it. so if you have 1GB and 1GB on new card, you will still only see 1GB.
I also purchased the 6850 and it is awesome. I think 2x6850's xfired is better than the 7850: Video Card Comparison - GPUReview.com
(in link, click "multi-gpu" to see what two cards would output)
but this is just on paper, I am not certain if it holds true in real world setup.
Also, I think the 7850 has a higher clock speed and this would be prefered. just don't get a 6870 if you do xfire as it will downclock to match the 6850 and don't get a 7xxx card unless it is the 7850 or higher. the 7770 is ok, but still not close to 2x6850 or the 7850
*also, for a laugh, check out the 7970 ghz edition on that site lol