Unless you have a use for the older processor (another build), or a buyer for the old one, and can get a great price for the new one, it's not a monumental upgrade, and you possibly wouldn't notice the difference.
(I'd first check that your mb revision number (1.0, 2.0, 2.1, etc..) supports the Sempron series of socket "A" processors, as many older boards, including some KT400A boards, definitely do not...)
Note: you can usually find good socket 754/Sempron 3000+ mianboard/cpu combo for not much more than $140, and that *would* be a noticeable improvement, as a socket 754 rig is generally a bit faster than a socket A rig...!