I'm not uptodate on this stuff since I work with laptops only these days, however...
Only games are demanding, any card will handle the other stuff fine (but check Vista capabilities of the card).
You probably have an AGP port on your motherboard, this will result in faster performance than PCI (but you may not notice the difference unless playing games).
Assuming you have a digital input on your monitor, aim for a compatible digital output on the graphics card. Unfortunately, different standards exist that use the same connectors (and may fall back to analog if incompatible) so this is worth checking carefully. If you don't have a digital input on your monitor, you must ensure that the card has an analog (i.e. vga) output. Google DVI for more information