This comparison chart may be a bit more detailed than you're looking for, but it will help you sort out the features of the various chips. Click on "View all products":
http://www.amdcompare.com/us-en/desktop/Default.aspxAlso, if you haven't already done so, browse through the AMD website for additional information:
http://www.amd.comPersonally, I have no preference between AMD and Intel. Both are good products. Your choice there. I wouldn't hesitate to buy a computer with an AMD chip.
To generalize...
If your computing needs lean toward just web browsing, email, word processing, and some photo editing and music downloading, go with the Sempron. It provides good performance, and is cost effective. (If you use Linux, add 1 gig of RAM, and you have a fine box indeed.
If, in addition to what's listed above, you're into gaming, video downloading, and more "advanced" tasks with a computer, you might look at the Athlon family of processors.
Is the Athlon 64 that much better than a Sempron?
To answer your question directly, no, both are fine. One's not "better" than the other, there are just different expectations from each family of processors.