I found a website someone made years ago that is no longer being maintained about how he upgraded his Dell GX1 to a 1.4ghz P3 and in it he says that he had to DOWNGRADE his BIOS to get it to work. I will be going through this soon with my GX1's, so don't assume that your BIOS needs upgrading to get it to work with the new CPU.

And if the instruction sheet for your slocket adapter doesn't specifically state that Tualatin is supported, don't try fitting a Tualatin CPU to it.