First, your RAM is already maxxed out and it is only DDR. See:
Secondly, PCI-E 2.0 is backward compatible with 1.0 slots.
The RAM on the graphics card doesn't matter as long as the card is compatible. As far as how much...if you can afford 1GB, great!
ATI or nVidia? That's personal preference. I have had both and was happy with both. Other manufacturers use ATI and nVidia chips and market them under their own name. Galaxy, XFX, EVGA, and others. Here are some mid-range boards from Tiger Direct:
You don't say how fast your processor is. You may be better off going for a new board or computer.
Hope this helps.
Thanks for using Fixya.