Two HD 4670 Faster than a Single HD 4850
 

At least in the preliminary benchmark scores; the 4670CF setup is slightly faster than a stock 4850 in 3DMark06 & Vantage. The individual scores were 54xx in SM2.0 and 65xx in SM3.0 tests, a 4850 on the same setup ended up with 49xx and 57xx respectively. The tests were done at the default (1280×1024) resolution, which is the target resolution for these new cards.

Since 3DMark06 is no longer the most current benchmark to gauge the performance, so we've got some Vantage numbers too; 73xx for the 4670CF and 72xx for the 4850. Again a slight edge to the 4670CF over a single 4850. The 4650 sibling will end up a good deal slower, aptly it relies on software CrossFire and comes without the CrossFire ports.

Single HD 4670: €70 * 2 = €140 , Single HD 4850: €130, you'd be quite stupid to invest in two HD 4670 cards...

http://gpucafe.com/2008/09/4670-cf-faster-than-4850/