How does my hashrate directly effect the coins I can / cannot mine?

Hey all,

So I have "a bunch of gpus"^tm and can produce about 100Mhash/s alone.

I wanted to do some tests on solo mining and wanted to know how my hashy-mc'goodness will effect the coins I can and cannot mine, based on that coins difficulty.

I know that I would not be able to mine something like Doge since it's difficulty is off the charts but at the same time, realise that there wouldn't be much profitability in it for me by mining an easy coin like CRC..

So is there a formula / algorithm out there somewhere that I can use to select coins with an appropriate level of difficulty for my hash power? What do the pool admins use since they would have this same problem (gigahash mining an easy coin would be a very bad idea would it not ? )

Thanks in advance for all your help!