i386 will abosultely work(but get the i686 livecd instead if u can), the athlon-xp one may or may not work on a regular athlon, but chances are it probably won't.
Gentoo is a little tough on newbies cus you have to do every aspect of the installation manually, partition with fdisk, format, activate swap, install bootloader, edit configuration files manually, extract packages from the cd, mount and chroot the new root partition, so on and so forth.
But fortunately someone has written an installation script, I havent tried it before but you can find it here:
Imo Slackware is actually a simple and straight forward distro, unlike some other distros it's not overly customized, however like gentoo it can be tough for newcomers to linux because a lot of things have to be done manually(but at least it comes with a functional installation script).
As for the boot loader just install grub/lilo in the mbr and have it load xp and linux.