TL;DR - Expensive cards run faster, ATI/AMD > Nvidia for mining, Radeon 5830 most efficient in their testing.<p>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.
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.