TL;DR - Expensive cards run faster, ATI/AMD > Nvidia for mining, Radeon 5830 most efficient in their testing.
It's a pity this article didn't look at a few other cards which would have been very competitive, namely the Radeon 5850's and 5870's. I've found my 5850's do very well, especially considering the performance/watt ratio.
This, of course, is making a big assumption: that the difficulty factor will stay the same. It has consistently increased, making earning those bitcoins significantly more difficult.
Break-even for me occured at about 75 days for my 5850's, but also used a much lower difficulty (less than 5% of the current difficulty) for the beginning of that.
Valuations are still quite speculative and difficulty can increase by an order of magnitude. Still, if you're not using the GPU but still using the computer, you may as well throw in a miner as a low priority process.
It's a pity this article didn't look at a few other cards which would have been very competitive, namely the Radeon 5850's and 5870's. I've found my 5850's do very well, especially considering the performance/watt ratio.