I would forget about the 486 for Ubuntu.
I'm not sure what you mean by "or P2.5 GHZ", nor how much memory it has,
but the Ubuntu Wiki has a page for installation on low memory systems
which they further specify as "(Pentium II and III Processor, 32-256 MB
RAM)". That should give you some idea of what you need for a regular
install. If they have any clear listing of hardware requirements I missed
it.
If you have a P4 there should be no problem. Incidentally, there are
*lots* of free distros (Linux versions) available for download. They tend
to run just under 700 MB in size so they will fit on a CDR. Many are
"live CDs" which will run from the CD-ROM without actually installing to
your hard drive so that you can see if they play nice with your hardware.
Naturally they run faster once actually installed.
If you don't have a broadband connection to download them, perhaps a
friend would do it. Visit Distrowatch
http://distrowatch.com/
and read about the various distros and visit the distro homepages. These
distros are all variants of Linux, some tweaked to run on older hardware,
some for speed on newer hardware, some especially optimized for multimedia
or security testing or whatever.
Seems like every Linux geek in the world has his/her own distro
Ubuntu is fairly good. Some people love it and there is a friendly
community that likes to help newbies, which is important.
Good luck!
Andy