Pc video card help? use 2 of the same better performance?

my system is a dell dimension3000 desktop/win xp home ed. serve pack 2. 768ram, intel celeron 2.66 GHz. the video card i have is a nvidia geforce fx 5500 pci bus,(pci is the long white one below the cooling fan right?) (fx5500 outdated i know)-- but question is could i put another video card of the same type in, in conjunction with my current card to better performance of games? or could i put a better video card in with my current one. if so does it have to a NVIDIA card or could i use a different brand? or will that cause compatibility problems. if ya cant tell im not that comp savvy, but im learning, much dew to the good people on answers. so feel free to call me dumb if you want, i wont get mad:) but any help will be greatly appreciated and i thank you :)))