I would, Plus I would go into your bios a disable the onboard.
If you plug your GPU in, the On-Board will be automaticlay disabled. And no, you don't need to uninstall your drivers of your on-board graphic card. Those are mostly connected to your mobo's drivers, which you'll also erase if you do that.
How would I go into my bios with running XP?
I what order should I do everything?
I had a dell dimension 4700 relying completely on the onboard graphics. Installed a 7600gt and never had any problems. No disabling, no nothing. Considering the system wasn't really "designed" for upgrades, I doubt you'll have problems.
Why not try installing the vid card and if you get errors or no display, uninstall the drivers...