If you don't need the power (only do wordprocessing, surf the net, no gaming, no video editing), sure, go with the sempron. Otherwise, I'd suggest getting something with a little more oomph....
The socket 939's are pretty nice, say the 3200. Its not much better going to the AM2s IMO. Not very good value for money at least.
And you can reuse your RAM if you want too (I'm assuming your old Dell uses DDR RAM).
Also, it'll help if you've got a budget we could work with.