It's been years but I've used and like Vector Linux - the light version requries Pentium II and 64MB RAM. The standard version requires a P3 and 128MB.
Vector is Slackware-based and actually pretty neat. With 512MB RAM I'd kinda keep my expectations low, though :)
I've run Vector Linux on a Pentium 4 without issues.
You will want the light edition, which you can find here:
http://vectorlinux.com/downloads
It is not as easy to use as ubuntu, being slackware-based, however, it does come with a graphical package manager, is very fast, and works well even with old hardware.
It's been a while since I installed vector, but I recall it being fairly straightforward. But, you can always hop on the IRC (vectorlinux.com/community), or use the wiki (https://vector.ecosq.com/ic/HowTos).