you don't need to reinstall to have all that 4 desktops, i have them on one install, all you need is synaptic and task-gnome, kde, lxde between others
there is the basic kernel, bfs, pae and a64, apart from the basic one those add extra settings that most users won't find useful, as a example the pae will need a pae compatible cpu and your athlon with almost 10 years(or less maybe?) won't work with it
the a64 is for cpus with 64 bit extensions but won't make your installation 64 bit, only will use some specific features of 64 bit cpus, i am using one of those but just testing, i had kernel panics with a older version
the bfs as you can read in synaptic has some features added but is very specific, very few people mention that uses it and to be honest this is all i could find explaining
http://ck.kolivas.org/patches/bfs/bfs-faq.txtin general terms, if you have older kernel and not much ram the basic kernel, usually called vanilla is your best option
if you have time you can install all 4 kernels and boot from each one a couple of times(first time it installs some modules, 3 or 5 minutes first time for each new kernel, only the first time), if one doesn't work restart machine and select another to see how system behaves and if you see improvements on boot speed and other details
in general, the best you can do to improve performance is add more ram, use a fast hard disk and disable the services you don't need in pcc but this won't help you alot in most circumstances, just will speed up boot time