Ubuntu 10.04 LTS (supported until 2013) takes 130 MB on boot. can go down to 110 with some tweaks
Lubuntu is the lightweight in the familly. takes about 80-90 MB ram on boot.
Kubuntu 11.10 (the new one comming out in October) will have low fat version for odler maschines.
Ubuntu 11.04 (in repositories) Ubuntu 10.10 and higher -- will have Unity 2D for older systems (i.e. unity with no compiz - looks the same).
Of lighter and user friendly distributions you have Linux Mint Lxde (uses L
E same as Lubuntu).
The realyl good, stable, light distribution is Chrunchbang. Don't be averted becase of their strange humor (making maschine go crunch, bang!). It's actually a solid distribution based on Debian Stable. But with some tweaks to make it easier for users to install and use (it comes with XFCE and Openbox). Xfce is like in Xubuntu only uses a bit less resources here. Takes about 80-90MB on boot i think. Openbox is even lighter on resources but also a bit strange/minimalistic.
If you have 512 MB you can easilly go with Xubuntu and that should leave you plenty to spare.