Well, you could try with synthetic benchmarks. They will give you some idea. NVIDIA'a CUDA toolkit for example contains some programs which are run on both CPU and GPU and it can be used to compare how long it takes to execute them on each platform.
If you need to do it only using GPU datasheets, you can do that too. For example, here's the page for my GeForce 9500 GS. You can find information about number of processing cores there. Processing capability is proportional to GPU frequency and core number. Some cards even have GFLOPS number too. There was a document with more uniform detailed descriptions available for each card, but I can't find a way to get to it right now. Maybe it's in CUDA toolkit downloads?
There's also the compute ability level which shows which features each card provides, but if I remember correclty, it doesn't make a direct impact on calculation speed, only on precision and instruction set that may be used.
I'm not up to date with information about cards from other manufacturers, but I expect that thare are ways to get the data at least for AMD and Intel too.