depending on your mb, the sempron at stock speeds will be significantly faster than the old pal 1800
should be much better for gaming for one thing but the barton core mobiles and regular chips are pretty good value too if your mb/cs can handle them
overclocking the sempron would also be a help, again, depending on the mb
I changed my daughters 2100 t'bred to a 2500 sempron and they are pretty much similar in a non overclocking setup
you should check the mb manufacturer's site for the max cpu you can use and check that your bios is up to date for it, the semprons default to 166fsb