I have an old PC based on the Athlon 64 x2 3800, with 2gb RAM and Windows XP. My once great Radeon x1950 Pro couldn't run some new games due to software incompatibilities. So I invested in the 6670. Anything with more grunt would have been wasted on my old system.
The result is that I have an up to date card that is completely software compatible with all the latest tech. My system is still CPU limited, but I have got a performance boost and can now play games like Fear on higher detail and resolution settings than before. And I use less electricity in the process - this card is more graphically powerful, but does not even need it's own power connection, drawing all it needs from the PCI Express slot.
Paired with a new Llano this would be an excellent setup. If I was building a system around something else, like Bulldozer FX or the older Phenoms and using an HD screen, then I'd be inclined to go for the 6770 instead. More oomph for all those pixels.
N.