Here's another spin on this from TG Daily. Kinda interesting!!!
ATI Radeon 4850/4870 mix and match with Radeon 3850/3870 PDF Print E-mail
Hardware
By Theo Valich
Monday, June 23, 2008 00:01
Mountain House (CA) %u2013 If everything goes according to plan, we will see AMD%u2019s new Radeon GPU launch this week. But there is more news to this announcement and we are thinking about one special item you are unlikely to hear about: During AMD's Cinema 2.0 schmoozapalooza in The Americano restaurant in San Francisco we learned that your Radeon 3850, 3870 or 3870 X2 graphics card will hold extra value: Don%u2019t get rid of your old card just yet.
A source close to AMD told us that there will be %u201Cinter-generation compatibility%u201D between the Radeon 3800 and 4850: %u201CSince they're both DirectX 10.1 parts, there is no reason why those two should not work together. (%u2026) We will probably not qualify this interoperability, but who stops you from trying?"
It appears that ATI did not integrate a driver lock or something else that would stop you from combining a 3870 card with a 4850 or with a 4870 model. Just keep in mind that these two cards need to include the same amount of memory. If you mix and match a 3850 with 256 MB of memory with a 4850 512 MB, the 4850 card will default to 256 MB. If you wonder whether you will be able to combine a 3870 X2 card with a 4850 board, our sources indicated that this scenario should work as well.
There is nothing not to like about this interoperability. I personally believe this is a big win for the consumer: How often do you have to throw you old card away when a new generation is introduced? Usually, always (well, at least, if you are among those who try to always have the latest and greatest.)
Here's the link: http://www.tgdaily.com/content/view/38056/135/
An older article from Legit Reviews mixing 3870 and 3850: http://www.legitreviews.com/article/663/1/