I would get a HD 5670 512MB if you can spend around $90. It basically consumes about the same amount of power as the older HD 4650, but is roughly 30% more powerful than the HD 4670.
The HD 5570 uses even less power and it's performance is roughly equivalent to the HD 4670.
I'll assume the HD 5450 is a near equivalent to the HD 4650.http://www.xbitlabs.com/articles/video/display/radeon-hd5670-hd5570-hd5450_6.html#sect0
This is a gaming review of the HD 5670 512MB:http://www.xbitlabs.com/articles/video/display/radeon-hd5670.html