I've installed my new HTPC2 with the ATI5450.
The PQ seems to be identical to the ATI5750.
The only difference I can tell is not in the PQ/SQ; rather it is the fact that my ATI5450 does NOT have a fan and so the GPU temp can get pretty hot (50c) compared to the ATI5750 (29c), which has a fan.
I'm thinking about getting a GPU cooler for my 5450.