Going from a MB that supports P2 to one that supports a newer P4 will definately require new RAM.

How's the video on the PC? Is it onboard? If it's an AGP card it's likely to be AGP X1. Is the new MB's AGP slot X1 compatible or is it's video on board?

Will your old power supply be powerful enough to run all your new components?

Will you have enough cooling for the new stuff?

Do you have the necessary equipment to properly static-ground yourself before overhauling your rig? (hate to see you short something out and have to spend more money).

What OS will you be working with and, while it's highly unlikely that it won't, will it accept and run some of the older components you'll be salvaging from the old rig?

Perhaps most importantly, since you're using a proprietary PC, download Belarc Advisor (it's free), run it, and take a look at how many of your installed physical components come up saying "Gateway" next to them. Do you know where to get the appropriate drivers for those intalled components for whatever operating system you install on your new rig? (should look into it anyway, it's always good to have access to your drivers even offline ).