You can't always judge cards based solely on their clk speeds.There are other factors which must be taken account of:
1.Radeon Cores/Cuda Cores
3.Memory bus width.
These are the most important parameters in judging a GPU.
Radeon cores musn't be compared directly with cuda cores.For ex 192 cuda cores roughly equals 800 radeon cores in terms of performance.
Higher ROPs result in better performance for both AMD and nVidia.
Memory bus width along with memory speeds determines the memory bandwidth.
Theoretically Bandwidth=Bus WidthXMemory speed.Higher is better.
Higher Core speed results in higher performance for same class of GPU.
Ultimately this will only give you a rough idea on how two gpus compare.For more specific comparisons look at the benchmarks.