There isn't really a 'system requirement' for a graphics card as such, you just need to be sure you have the right motherboard slot (i.e. agp/pci-e) for the graphics card you're getting.
Graphic card ratings across companies can't be compared numerically, 7series Nvidia Geforce is compared to the X1 (11) series Ati Radeon ....
It gets more complicated with their internal ratings. So 7 for the generation then you have ratings (as in power) say 7800, 7600 etc... But a higher number doesn't mean a better card. Just because a x1300 (in theory 11300) has a higher number than x800 (10800) doesn't mean it is a more powerful card.
In the ratings, both companies use their second NUMBER as 8 usually to represent their top cards, 6 to represent mid range and 3 and 2 for Ati and nvidia respectively for their entry level models.
check,
http://www.anandtech.com for details on most graphics cards.