Depends on the mobo/chipset. I have an Asus P3V4X. It's VIA chipset hates CAS3. I saw a significant performance boost going to CAS2. No benchmarks to prove it, but I'd say it had to be around 10% because it is enough to notice in general use.
Other chipsets won't give as much difference. I recommend CAS2 becuase it isn't all that much more expensive. Go get some good CAS2 in the For Sale/Trade forum. You can get the good stuff there cheaper than brand name CAS3 on most web stores.
Good Luck.