You can get really reasonably priced 128Mb cards now and if you're into CAD, you will see an improvement.
Just remove the old card, fit the new and on first boot-up, go into "Safe Mode". This will bypass your existing graphics drivers. You can then remove the old card via Device Manager, reboot and let Windows find the new card.
At this point be careful. Read the documentation with the card, because it may require you to tell Windows to ignore the card and then use the Graphics card manufacturers own installer to load the drivers correctly, or it may require Windows to do it's own thing. But either way, follow the manufacturers instructions.