If you want to pair a GPU in crossfire with your 5800K, then the best card to get would be the HD6670. I'm not sure whether the GDDR5 version in this case would help or not. I know that when compared as discrete solutions, the GDDR5 version of the 6670 is worth the extra $10-20, however, I get the feeling like in hybrid crossfire, the memory bandwidth of the lower speed card will dictate the speed of both. The DDR3 version of the card has about the same memory bandwidth as 1866 speed system memory. It would be worth investigating whether hybrid crossfire would benefit from a card with higher bandwidth or not.
When cards are listed as being "crossfire compatible" or "not," they are talking about whether the card has a crossfire bridge connection ...
That's a picture of the back side of an Asus HD7950. Notice what appears to be a pair of short slot connections at the top of the card. Each of those is a crossfire bridge connection, which is used for connecting multiple discrete cards together. In the case of hybrid crossfire (with the APU), this bridge is not used, nor needed. You just need to select one of the video cards that is compatible with hyrbrid crossfire, which, as far as I know, is the 6450 - 6670 series cards.
With an HD6670 in the mix, you should be able to achieve GPU performance in the ballpark of an HD5770/6770, which is nothing "special" by todays standards but may make the most sense financially. In order to beat that level of performance by a margin large enough to be worth it, you'd have to look at options in the $150+ range (GTX550Ti, HD7850, and GTX660).
The APU solution gets WAY over suggested by AMD enthusiasts, and typically for all of the wrong reasons. The APU solution is actually at it's best when used as a low cost workstation type machine, that can take advantage of openCL in various productivity apps, and/or can take advantage of 4 cores in media or content creation apps or encoding and through general multi-tasking that is common when using a machine for actual work. As a gaming rig solution, The APU is usually hard pressed to produce a better result than a similar costing discrete build except when the budget tightens down REALLY hard. As a productivity chip, A trinity based machine trades blows with a similar costing i3 build in almost every possible real world (application) comparison, and can be used as a light gaming rig when the work of the day is done. THAT is where trinity makes sense.
EricEdited by mdocod - 10/20/12 at 5:23pm