for your older system i would reccomend **** small linux. It comes as a livecd but can be installed to your hard drive or usb pendrive,and its only 50mb!! Puppy Linux is also a good choice it has gone through a lot of development recently.
For your newer computer i would reccomend ubuntu if you just want to make things easy.It can resize your windows partitions,which i've done safely, and should support your raid devices although i'm more of a software person not a hardware person so someone else will have to help you if ubuntu dosn't already support it.
Ubuntu is based on Debian which i love but without all the hassle.It all comes on one cd which saves a lot of time downloading, and adding new packages is easy. Mandriva is also not a bad choice but ubuntu is just simpler and just works in my opinion.
www.damnsmalllinux.com www.ubuntu.com
lemme know how it goes and don't be afraid to ask quesdtions,especially about your new computer, if you get stuck don't risk your data just ask!!