I suggest to go for an XFX any day.Reason is simple- XFX's availability,after sales support, quality, and overclockability. I agree with KHUBBU.Undoubtedly Gainward offers superlative performance,but certainly not BIG.
XFX wasn't actually BIG, it marketed its products under the BIG banner a couple of years age. Now, coming back,BIG cards don't offer many features, and overclockability.6600 GT chipset is quite powerful and won't be obsolete for 2 years.It WILL be able to play all future games albeit at a low resolution and medium quality setiings.
As for Gainward golden sample's availibity,its AGP variant is perhaps not available. Please dont go for BIG though an XFX costs few bucks more. One of my friends has a BIG 6600GT,it doesn't deliver good performance in FEAR.Another friend has the same card from XFX,but FEAR works better on his PC.They have identical config. If you want to save good buck,then go for a non GT XFX or Gainward 6600. It will last you a couple of yrs. Costs abt 6.5k
-Kaustav