lol, thems fighting words.
Really, they both make good cards, it all comes down to one's own personal experience. I lean towards ATI right now, others lean towards Nvidia. No matter what card you pick, a few more bucks gets a better card from the opposite manufacturer. Then a few bucks more gets a better card from the other. etc...
Also, are you talking AGP or PCI-E? Would you ever want to run in SLI or Crossfire? Or in other words run two cards in the motherboard?
If $300 is the budget, run a search on newegg and browse around in that price area, compare the pipelines, ram speed/type, clock speeds. Then head over to
www.tomshardware.com and look at some of their charts:
PCI-E:
http://www.tomshardware.com/2005/12/02/vga_charts_viii/page18.html
AGP:
http://www.tomshardware.com/2005/07/05/vga_charts_vii/page4.html
For the most part, if you don't get a cheap budget version card, but a "Pro" card or one of the higher-end types, you should be fine either way. Or get an "OC", or, Overclock version.
If you spend $300 or even more, take a quick survey of your power supply and post the specs here, you don't want an underpowered power supply when running a fast video card.