You need to consider which type of card you want first.
Would you prefer an Nvidia or and ATI.
All manufacturers use one of these two 'chipsets'
Once you have decided that the rest is easy.
On occasion one manufacturer will squaeeze a little bit more out of the same chips as the next manufacturer but unless you are a millionaire nut who upgrades to the best of the best of the best every 12 weeks, then you aren't going to see or notice a huge difference from manufacturer to manufacturer.
I have an ATI 9880XT which is probably just out of the $300 and it works very well for Doom 3 and other games.
I have used Nvidia cards in the past too though and found them to be good solid cards.
I would go to a store with the $300 and ask the nerd behind the counter what he rates at that price and go with that. Then go to another store and see if you can get the card the first guy recommended for cheaper!
