The Mainstream segment quickly falls out of real gaming performance. The HD 5670 will handle many games on a single screen, assuming you mainly play older games (which rely less on Shader performance) or willing to make the quality sacrifices in newer games. The 5670 can handle older titles well on a single screen, but can only handle Eyefinity in a few rare instances. Additionally, I had issues with using the HDMI connection for Eyefinity. That screen always had a black border around the image when using Eyefinity. My guess is it's an issue with the image scaling and expected overscan on an HDTV. Whatever the reason, I can't recommend an HDMI connection for Eyefinity.
The 5670 is a hard sell in that for a few more dollars and a single power connection to your PSU, you open up much more performance from the 5700-series. But, if you're on a strict budget and have a lower resolution monitor, the 5670 might do you well. It would also provide superior performance in an HTPC with good ventilation (as long as you don't need a silent GPU).
The 5550 is a much more interesting card, as many vendors are now offering fanless options. The 5550 offers solid performance for a silent GPU. Pairing with an HTPC and a 720p HDTV would be close to perfect. It would even work well on a 1080p set assuming the viewing distance would minimize some of the quality reductions the card would necessitate.
I won't be doing any more testing on the Mainstream cards, but instead will continue to focus on the mid and high-end cards. These are more in line with our readers and their needs
As I wrap up testing on the 5850 and 5770 in CFX, I will be producing charts that compare performance to total price. This will be the backbone of our forthcoming Eyefinity Buyers Guide.
If you have any interest in Eyefinity, or game in anything more than a cursory fashion, I would recommend the upgrade to the 5700-series. The one downside of the 5700-series is that it does require a power connection from the PSU. It would be nice to get the 5700-series performance in low-power card. Maybe next generation
The 5550 could provide the backbone for a solid HTPC, considering the fanless options now available. I really can't recommend the 5450 for much of anything. It's gaming performance is poor, and s fanless 5550 provides an HTPC with considerably more power.
The 5670 is in a tough spot. It's not a fanless solution, so it won't fly for the hardcore HTPC crowd. A few more dollars and electrons gets you into the 5700-series. The narrow niche I can think of for the 5670 might me something along the lines of a 2nd PC, Small Form Factor PC, or something for your mother or girlfriend (who possibly plays WOW) that you might use from time-to-time.