The Socket 775 CPU is the latest edition, and the motherboards for these are more expensive than the socket 478 models, but in tests the socket 775 mobos are slightly faster, they may also include things like Intel Extreme Audio, which knocks the socks off the old AC97 audio, SATA2 and DDR2 memory.
The socket 775 chips also have no pins, so are less likely to suffer damage should the person assembling your pc not put it in the socket correctly.
As always in these discussions, my advice is to get the best you can afford, but price up the whole Motherboard, CPU & memory bundle as new technology can be significantly more expensive.
ps Why move to Intel? Get an nForce 4 motherboard with an Athlon 64, then you have a faster, cheaper system than Intel can provide, and your ready for 64 bit Windows next year
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