even at the same clock speeds as an intel processor, an amd processor will work just as well as an intelk with a 1ghz advantage. thus, in my opinion means that that sempron, we definately beat the crap out of that celeron, unless the celeron has more than one ghz higher clock speed than the sempron.
2ghzsemp=3ghzcel performance wise
often you can overclock the semprons higher too
and, of the motherboards you picked, the one with the sempron also has an agp port. so id take that into mind aswell, as there are now a shit load of good agp cards that are just devaluating and ripe for that budget system of yours.
also just noticed that the sempron was a socket A... this means it has a much lower fsb than the socket 754 ones, the celeron may actually be a faster cpu... but since it doesnt have an agp port id pick the sempron, if u plan on doing any kind of gaming whatsoever at least... if not then the celeron with a cheapo pci graphics card will do fine for any basic app and get this!! web games from www.free-arcade.com
here is my thoughts if you are looking to spend around 250 on a board, cpu, and memory all these can be purchases at zipzoomfly.com and they offer free shipping. i use these guys on all my online orders and never had a problem. here is the item and price for the items. these are rough prices on the sales tax from where i live so it might be a little higher where you live or lower.
There are socket A Semprons (which are essentially relabeled XP series processors), and socket 754 Semprons, the latest models which are 64 bit capable; the latter are much better performers than the Celeron D models, typically outperforming the Celeron D even at a 1.4 Ghz clock disadvantage....