I purchased a 5570 the other day which although a low end card is a good compromise between power consumed and Mhashes/sec.
I'm now looking at a medium level factory overclocked 6870, which again has a good Mhashes/watt rate. It's about 4 times faster than the 5570 (which will go in my winbox and keep mining)
If the new card pays for itself then it might be time to move to the big guns.
I wonder if that would make an interesting blog, starting with $50 for a low end card and working your way up as each one pays for itself... it's a pretty conservative strategy but it could be fun.
