Ah, I thought my information was a bit dated. A couple of months back, AMD with DDR RAM was top... Unless people were BS'ing
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I would go with Manasecret's advice. If you really want to get the best out of your games, you may want to start with upgrading your RAM. And if you've got enough cash after that, definately try getting a better GPU. Look into the 128MB cards. The best now now is the Radeon 9700. But I guarantee that in a short while, Nvidia will top it with something better. So wait till then, and the price of the current cards should drop a bit.
To get an intensive game like Doom 3 running at a decent rate, you may have to get a very good GPU. I don't even think the top ones at the moment will be able to run it at its optimum, but they will definately do a more than adequate job.