If you want to know if your processor is going to run a reasonable portion of PCSX2 (there are games that run really well on most things and ones that dont run well on government super computers, this is for most of the rest) games at mostly full speed (there might be some really heavy sections that original PS2s slow down in and a lot of computers as well) then use this info
http://cpu.userbenchmark.com/
In the search box on the top right corner type in the CPU you are using. Look for the SC average score. (Single core is used because emulators rely more heavily on single threaded tasks over multi threading (even when MTVU is used is does not make the load go 50/50))
a good score is 100pts -- this is roughly equal to a stock clock 2500k. (this score was used after looking over
FFX-2 CPU benchmark thread the testing results and methodology are all in the thread go over there and give thanks if this post helped you) Basically this is a reasonable performance level to achieve the above goals. That said.
If you can overclock it can help (the CPU user bench has scores for what overclocking can get you)
If you are using an integrated GPU then aim for a higher score since either your CPU will have to carry your GPU or your GPU's heat will limit the maximum performance of your CPU. 10-15% is a good offset for a bad integrated GPU (assuming it runs at all)
All of that being said. Really the simplest answer is that PCSX2 is free so if you really are curious what your performance is then just run the game you want to try out.