Your budget for a GPU is $500? The answer is clear
7970 ghz edition! Just make sure you get one with a decent cooler on it and not the reference on as it's quite loud. I don't think there's a card in existence thought that can run Metro 2033 on very high settings even at 1920x1080. Now, if you have intentions of overclocking the card you may want to go with one of the original 7970s as they're likely to be binned with better parts than the new 7970 ghz edition cards and can usually hit the same max frequency as the ghz edition.