I agree... If you have the Sempron already, dont waste your Money.
I got several PCs, among them a Sempron 64-3100 and a P4 3.6Ghz, and the Sempron has a Rad9800Pro, the P4 has an x800GTO and to be honest, there isnt really all that much in it!!!
Ok, so the P4 gets a few FPS more in some games, but really, the money I have thrown at the P4 is pretty disgraceful.
Whats even more worse, is that you get that Sempron clocked and it will beat the P4.
As for this HyperThreading lark, I find it a little bit poo to be honest. So far, I have seen nothing on the P4 that my half decent AMDs cant do better... The Sempron is not really up o heavy multitasking, I admit that, but the others ( NewCastle, ClawHammer, Winchester & x2 ) are all seemingly quicker than the P4 when loaded up with lots of stupid apps, and the only thing thats helped the P4 chug along is that I have thrown a quick HD at it, cos otherwise, its a bit of a fairy.
Its saying a lot to me, that I have a P4 that is only about 3 months old with 2GB of RAM and 2x200GB HDs and I am getting rid of it and using a Newcastle 3000 in its place cos its quicker ( Overclocked mind you, but still )