XFX’s 8600 GT XXX takes the 540 MHz reference frequency to 620 MHz, and the memory from 1.4 GHz to 1.6 GHz, which will help it in benchmarks against the 8600 GTS XXX and HD 2600 XT.   [H]ard|OCP takes all three cards into their benchmarking suite, to figure out which provides you with the best bang for the buck.  The benchmarks are very tight, but the price range is not.
“A GeForce 8600 GT, a GeForce 8600 GTS or a Radeon HD 2600 XT, which is the best value?. We will compare a brand new XFX GeForce 8600 GT XXX Edition with a XFX GeForce 8600 GTS XXX Edition as well as the Radeon HD 2600 XT in current games.”

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