Both of those cards will work for photoshop. I would go for the ATI card since it is newer, probably performs better and ATI cards have been known to have better colors.
I would go with the 8500 gt if you have the money, its 38 after rebates right now the same as the 7300gt. It also out performs the 7300 gt by about 30% on the 3Dmark score.
fyi : you said you wanted dual monitor, just making sure you have at least one vga output and not 2 dvi outputs for your monitors because if you didn't you'd have to get an adapter. But if you have 2 vga or 1 vga and 1 dvi your fine.
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