First, I am not really a computer person. I have an older, second PC, an HP Pavilion 6630 with Windows 98SE that I started using again. I use it for internet surfinga and making CDs & mp3s for music and old radio shows. I have added 64mb ram to give it 128, replaced the CD burner and when the PC was fairly new, added Sound Blaster.

What would I gain by adding the SP upgrades? any particular one? I would appreciate any guidance here.

win98se was a great OS, but is not going to be in deployment anymore.
that means no longer updates (except the critical ones) will be released for it. meaning at the moment you can't install wmp10 and in a future... well we don't know...

if you maintain your system updated using WU there won't be any problem on sticking to it, specially a 128mb machine. if you surf the inet, let me recommend you to use a software firewall and the firefox browser, that is the only weak point i can see. good luck

ps: forgot to say welcome to the forum

even an old computer would benifit from the sp
it adds updates n more
it would make it look more modern
98 support will end on june 30th 2006 but 98 is used by many even to this day so ya you should use the sp.