There is no CPU called the AMD x4, there are AMD Athlon 64 X2, which are dual core chips, there are Phenom's, then there are a lot of in between variations on those names.
If your motherboard has an AM2 socket, then you can't put a quad core, only if it hs an AM2+ socket could you do that. I would recommend an Athlon 64 X2, probably 2.6GHz minimum, that would be the AMD Athlon 64 X2 5200+, I have an Athlon 64 X2 6000+, 3.0GHz dualcore.