depending on the video card you have, which is probably discontinued and not supported anymore, you might still be able to purchase video ram for it, but since these are proprietary and card specific, prices will be very high
all you need is a good 2D graphics card for internet that supports relatively high refresh rates at decent resolutions for eye comfort
Since u have an old system, you can probably find some used stuff for cheap in the For Sale/For Trade Forum or new old ones on pricewatch.com.
I can't reccommend anything because it depends on what you are looking for and how much you want to spend.