The 64 series is almost pointless at this point. Yeah, they are supposed to have an increase in performance...but it's not worth the extra cash. Just get an Athlon XP for now, you will be fine for quite some time...like I said, get something around the 80 buck margin, and if you feel like you need to upgrade, which hopefully won't happen in the next year or so...unless you are really that greedy, than that will be fine.
Now some kid will come in here telling you that the 64 is the best thing in the world even though there aren't any programs that are 64-bit yet...he just likes to spend money...kids are idiots these days =\
Remember, the most expensive isn't always the best. Hardware is far beyond the reach of software at this point, so getting the older stuff at the cheap prices nowadays is golden.