Your Gateway already has a brand name processor(CPU). But, unfortunately for you, it is very old and you won't find a faster CPU to put in that baby that would give you a noticeable increase in performance. If you got windows 98, then you prolly have an older and slower processor (CPU).
My suggestion would be to hold back for awhile on new purchases for your reef and save some dough for a new tower. Not necessarily "new", but you can have someone build you a pretty nice system for about $400.00 - $500.00. You could also go and buy an E-Machines tower and hook it up to your existing monitor/keyboard/mouse/printer and go that route for about $300.00!
If you can't afford any of that right now, then an increase in RAM (your computers memory) would be a good choice. You can find older RAM sticks and someone to install them for you at rock bottom prices right now.
Either way you are going to have to get rid of that old Gateway!
If you lived out here by me in the QC, then I would be happy to take a look at your system free of charge and make you some better recommendations! But, Phoenix is a loooong way from here. I'm closer to Oro Valley/Tucson, than I am to PHX.
*edit* - I forgot you mentioned video card. Many older Gateways and the like use shared memory from your system for an on-board video card. You would have to find a smaller PCI Video Card to fit in that baby. Depending on which model of Gateway you have, it might even have an Accelerated Graphics Port (AGP) which you could use for a better video card. Even if you update just the video card, it would prolly not be compatible with a future computer purchase and you have to trash it.