9800 for $199 (MSI RX9800 with 9800XT core)
6800GT for $389 (or
$340 on Provantage's website)
X800XT for $500+ (the fastest card period)
Of course before you invest into a fast videocard, consider your other computer components like CPU, MOBO, RAM
the minimum you should have is Axp2500+, 512mb
Before investing into the latter 2 cards I've mentioned, it'd be best to have A64 3000+ or P4 3.0ghz+ and 1 gig of RAM.
Of course the videocard is the boss when it comes to gaming above all other components, considering your system meets the minimum requirement I stated above. Otherwise, you are just dropping extra $$$ on a videocard for nothing as your system will not take advantage of it.
No system out there now will play doom 3 at 1600x1200 4AA/16AF anyways so you cannot get to top setting because videocards do not have 512mb ram yet, cpus are not at 4000+ rating and 1.5+ gigs of RAM is not easy on the wallet.
If you tell us more about your system specs, we can help you find the videocard which will suit your computer the best.
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