Which P4 cpu is it ?

Which live cd is it ?
Have you got the cd yet ?

i686 is for 32 bit processors
x86_64 is for 64 bit processors


Have you got windows installed at the moment ?

If you have then download cpuz from http://www.cpuid.com/cpuz.php

and install it.

Follow instructions from http://www.lytebyte.com/2009/08/04/h...it-in-windows/

Please don't post in bold writing, it doesn't make people answer faster.

The i686 choice is at least OK and probably better.

If your CPU has x86_64 support then either i686 or x86_64 Linux will work. If your CPU lacks x86_64 support then only i686 will work.

If both work, 512MB is low enough that the disadvantages of x86_64 for small amounts of ram might be enough of a factor to affect the decision (but 512MB is not so low that it makes the choice of i686 clearly best). 40GB hard disk is also low enough to notice the extra disk space needs of x86_64 but not low enough to force the decision of i686.

It depends on what you will be running on the system and there won't ever be enough information to make a well supported decision. Assuming either would work, one will be slightly better than the other and you'll just need to guess which.

If you don't want to even figure out whether x86_64 would work, i686 is the safer choice.