There's not a great deal to go on except that lesser quality chips are not allowed to be factory overclocked according to many reports. 'A' chips are generally 10-15% more expensive. Pins and power limits are good indications only. The Specs table in this threads original post is a good guide. Techpowerup keeps a fairly decent repository of GPU specs and of course there's always Google.
It's a bit of a minefield to be frank. To add to the confusion I've read from reliable sources that NVidia is offering three performance brackets to their partners. Standard chips, which can barely be OCed at all, some with medium potential and some that are especially great for OCing.
It ultimately comes down to research and doing what youv'e done. ie. asking the community.