I'll put my vote towards a Radeon 7770/Radeon R7 250X(Both the same, get whatever is cheaper, provided it's GDDR5)+PowerSupply.
A Corsair CX400 would be more than ample.
Or an Antec NeoEco 420C.
Try and get a PSU that's efficient, it would save you money on the power bill and it makes an allowance for capacitor aging.
If you want something cheap and nasty and very low-power, the Radeon R7 240 should fit the bill, just get one that is either passively cooled or has a massive quality fan.
Low-end cards usually have horrible and cheap heatsinks and fans which don't take long for them to fail.
Otherwise, there is the Radeon 6570 and 7570 (Both the same.) which are a massive improvement over the HTPC card that is the Radeon 6450. (Again, try and go for a passive card.)
Then you have the Radeon 255 which is not to far away, which is the Radeon 7750's replacement. (Both the same.)
Basically, the general consensus is, the Radeon 7770 is the best price/performance.