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.
i7-4790K @4.2GHz, GIGABYTE Z97X-UD5H-BK, 16GB GSkill RipJaws PC1866
Corsair 650D, Seasonic 1Kw Platinum PSU
Samsung 850 EVO 500GB, WD Raptor 600GB, WD Black 1TB, NEC 7200 DVDRW
ASUS STRIX GTX 980, X-Fi Titanium, Dell 2408WFP