Think of it this way, I used Ubuntu (currently using Firefox 3.6, Firefox 3.5 whenever that one fails, Opera10, and the now blisteringly fast (about as fast as Safari4, and faster than Chrome) Epiphany-webkit browser.
Ubuntu gets more stable and faster the longer you use it with internet, with no antivirus running and no visible security software. For many tasks, it actually works a bit slower than Windows (not sure about XP, but saw some benchmarks for Vista) - however, it has a smaller impact on the system, so the computer is less 'clogged' with stuff going on in the background. For me, I find that Ubuntu is simply much nicer to use, and has no problems with speed. Direct comparison? Well when I boot XP, and click to open a file explorer window, it appears almost instantly. When I do this with Ubuntu, it appears more slowly - it takes 200mS to perform an animation (it can 'burn' or 'shimmer' or 'fade' or 'fly' into view) and minimising is the same - my windows 'slurp' to wherever they're minimising - a bit like Macintosh style, 'magic lamp' effect. I don't mind a 200mS delay as it disappears, it's not slow enough to make a difference (I tried 100mS and it's too fast for me, and 300mS was too slow).
I get updates through my repositories for all the software, not just the OS - so managing my system is a breeze, without having services running to do the job for me.
Security updates? There was a problem with the linux kernel that took about 5 days to fix, upload and patch. With Windows, there was a famous DDOS attack earlier this year that uses a hole that is still unpatched after 7 years.
You pay your money, you make your choice.
Mine boots in 25 seconds - about the same as a stripped down version of XP I use to play games (but with no internet or services running - so that doesn't degrade or get infected).
For a 600MHz computer, not so sure - maybe you should try something lighter and faster than a gnome desktop - not sure what to advise really. Linux isn't always very good with hardware more than a few years old - things are changing, but they won't spend time fixing the backlog of computers that will be ten years old by the time all the bugs are fixed.