I recently wrote an article addressing this exact question.
http://forums.pcapex.com/showthread.php?t=31960
You also might want to look at this thread, I took a look at the Celeron VS Sempron Vs Athlon XP question there.
http://forums.pcapex.com/showthread.php?t=32747
The best buy for under 150$ is the AMD 64 3000+ or 3200+ on Socket 754. The DFI lan Party is by far the best board for this platform, commonly reaching 250-300FSB with good memory
For 156$ you can get a socket 939 90NM 3000+, the 3200+ version is around 200$ and just out of your price range...it should drop a bit after the new year though.
AMD 64 3000+ socket 939
http://www.monarchcomputer.com/Merch...ory_Code=AMD64
I would go socket 939 with your budget, MSI has a good board if you want to Overclock it a bit, and you can also choose to go with a PCI Express motherboard, something you can't do with socket 754. This platform is supposed to be capable of using the new dual core
CPU's due out late in 05.