I am about to upgrade from a ATI 9800pro with 128 ram and 256 bit color. In my research of which card to get, I can't help noticing that many Mid-Priced cards are only 128bit color. Why is that?
The 9800 Pro was the high end AGP model in the 9xxx series line up. You would see 512bit for a memory interface on some of the Radeon HD 2900XT models. The MSI HD 2600XT used here has a 128bit interface seen at http://www.newegg.com/Product/Product.aspx?Item=N82E16814127298
256 colors is the defaut for XP without any 3rd party video drivers installed and you are using those basic vga drivers provided in Windows. Until the drivers for your display adapter are on you still have to have some basics to work with.
The difference with the memory interface has more to do wirh bandwidth. While still seeing 128 for a number you also have to look at a PCI-Express 16x type bus over the AGP 8x/4x bus. The processing power on the newer cards provides more data streams then seen with AGP models as well.
Here's a comparison one of the newer mid range cards(MSI model used here) compared to the 9800 All-in-Wonder model there. The MSI HD 2600XT sees "Stream Processors120 Stream Processor Units ".