The top to bottom order of the cards mentioned are:
GTX 460
HD 5770
HD 5750----GTS 450
HD 5670----GT 240
I'd go with a 5000 series card or a 400 series card as their HD video rendering is better and I know the 5000 series cards can do HD audio and I think the 400s can also. Also of course the higher end cards need a more powerful PSU.
The GTX 460 is getting a lot of raves because of it's power to dollar value, though it may be more than you require.
Here's a good deal on one.
http://www8.ncix.com/products/?sku=54626&promoid=1223