I do notice a slight performance difference when going from plain surround to EAX4 in everquestII. I never notice it in COD though, even w/ my old pentium system. I run it at 1280x1024 4xaa, 8xaf w/ eax3 on my athlon64 machine and it never lags.
Full comp specs can be found if you click my little sig link.
If I was to blame anything I would blame dell mobo.