Athlon 64 X2 is AMD's dual core processor just like Intel's Core 2 Duo. Two Athlon 64 placed together make Athlon 64 X2.
AMD had caught up with Pentium 4 processors, but after Intel released Core 2 Duo's there is no competition. The only advantage right now is the price.
If you don't play games and are on a tight budget go for Athlon or else Core 2 Duo.