Go with the gainward. The 550 means that they run there memory bus speed at 225 MHz. Since it's DDR RAM it clocks on the rising and falling edge, therefore at double the data rate hence 550. If your going to blow the money on Ti-500 that's how I'd go.
Personally I'm running a Ti-200 right now. It still pumps FPS's at over 90 FPS at 1289x1024(max of my monitor), and only costs $147 Cdn (I won mine in a draw though which only makes it sweeter). Anyhow that's my input.
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Remember Dieppe
