I saw an ad for a used Intel P4 2.4, 512, onboard sound and lan for $300 USd. No monitor, HDD, CD, mouse, keyboard.

not spectacular, no- the chip's prob ~$130 and unless the mobo and ram are top-notch you arent really saving anything and could be buying somebody else's problem.

I don't know anything about chips and motherboards and what is good but here is the complete specs:

Intel P4 2.4Ghz, 533mhz FSB
512MB Rambus Memory
Gigabyte GA-8IHXP motherboard with overclocking function with
NIC, onboard sound, USB 1.1 & 2.0, raid support.

No, it is not a good deal. Not for me, anyway.

Of course, this depends on your idea of "good deal".

When I buy complete systems from people that around your specs, I usually get them for around $150 if they are in good condition with an 80GB or larger hard drive.

Of course, I then resell these systems for twice the price. So maybe if you can weasel something like $200 or $250, it would be better.

Do it piece by piece if you can...any buddies that know the tech side of things, and who won't rip you off?