I'm going to say Radeon 5570 as well. You go up to a 128-bit memory interface (as opposed to the 64-bit of the 5450); to me, that's a big deal.
For retro-gaming at 1280x1024, it should be more than enough. I bet you can find a bulk lot on Ebay. Second choice would be the 6570, but it's probably another $10 apiece at retail.
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