Let me ask a couple questions to get a better understanding of which X800 and which X850 you are refering to.
Which X850 are you refering to? Pro, XT, XTpe? The X850 pro is not worth the money as it is slower than a X800XL and costs alot more. The X850XT is a killer card if you can afford it.
Also, which X800 are your refering too. Do you have the R430 based plain PCI-e X800? Or are you refering to an X800se, X800pro, X800XT or X800XTpe? Alot of names, and knowing which is important.
If you have a plain R430 PCI-e X800 and stepped up to a X850XT, then yes you will see a difference as resolution and eye candy get cranked. 1280x1024 and higher with AA/AF, there will be a big difference. 1024x768 no AA, there won't be much of a difference. Anyway, it depends on the game and desired resolution and settings you hope to use.
If you had a X800XT AGP and went to a X850pro, it's a downgrade.
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