It's actually more than just video memory at play here. The VGA must put out the right mode but AGP is so dated that you will have to dig up such a card and then check if it can produce the needed video mode.
I usually had to settle for a PCI video card for such work. These are getting rare but how about a PCI card like this one:
I was told by a tech guy at Staples that to be able to use the full width of my widescreen monitor, I would have to upgrade my AGP card...from 64 MB to 128. Is this true? Is that the limit for this model (memory-wise), or will a card with more memory work better? What do you guys recommend?