1. 7950 has 3GB of ram, nvidias max out at 2GB ( so it is a bit more future-proof)
2. You might want to look at non-Ghz ed 7970, that's 25% extra horsepower for not really that much more money, for even more future-proof-ness.
It all depends on your upgrade cycle... if you go only once per 3-4 years, than pick a more expensive card. If you upgrade every 2 years, get the cheaper one
3. 7000 series are quite overclockable ( I run mine 1080Mhz vs 925Mhz default, that's 16% extra, with no overvoltage ! )
4. AMD/ATI seems to magically get more performance from their drivers in time, while nvidia gets almost the "max" at launch, so don't expect much more out of them in time.
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I did a thorough research before getting my card 1 month ago ( a Gigabyte 7970 )