No version of Ubuntu will work very well with that config.
Lubuntu would, I highly recommend it. It is much better then Knoppix or *censored* Small Linux.
"The hardware requirements of LXDE are similiar to Windows 98 (Maybe a little bit higher). An old Pentium II CPU is enough." LXDE website
Lubuntu will work with 64 MB RAM but recommends 128. So if your computer runs 98 well, it should run Lubuntu decent.