With ROHQ, Radeon cards do not support the overlay mode of Madvr. Whether that's something to consider only matters if you're actually using overlay mode. I do.
I think this is very personal. I've used a HD4000, currently using a GT450 passive and I've tested with my GTX580 as well. The GT450 is slightly faster than the HD4000 but not enough to use higher scaling settings or improve the poor deinterlacing performance on HD material. It does however allow me to use overlay mode so that is one of the GT450's benefit over the HD4000. Another important thing for me is that it better supports all the refresh rates I need (my TV supports every refresh rate video material can use).
With the GT450 I'm using the High settings
advised here. More information about configuring Madvr can be found in the excellent guide by 6233638
found here. Highly recommended reading!
The problem with my settings on the GT450 is if you let Madvr deinterlace HD material, it's not powerful enough so I disable that. I don't have enough interlaced material to care.
For upscaling SD and 720p to 1080p I find the settings I mentioned to be good. SD is typically too bad anyways for higher scaling algorithms; crap in, crap out so to speak. I guess you could opt to mask artifacts but I don't care much for the softer image. For upscaling 720p I found I don't really see a difference at 4 meters on my 50" plasma. I actually made some screenshots of several stills to compare higher settings from up close. Although I saw a difference, for some stills I couldn't decide which one actually looked better (some were obviously better up close) but more importantly at 4 meters distance I couldn't spot a difference at all on stills or on a running movie.
I also used my GTX580 for testing interlaced material. It works and it looks better so if that's important to you, you may want to consider a more powerful card. THe problem however is that its so GPU intensive I don't think any passive card can handle this properly. My GTX580 jumped to 100% with fans blowing at 100% within seconds. I believe Jmone tested a GTX660 and found it to be the minimum for higher settings but if this included interlaced material I don't remember. Do a search in the video cards section of the forums.
So unless we get a more powerful passive card that handles 1080i deinterlacing at higher scaling settings I see no reason to upgrade my GT450. But again, this is my personal opinion from the testing I've done. YMMV so don't take my word for it, you should do your own testing. The HD4000 can do the highest settings with Madvr, it will drop frames but you can find a few still images from different scenes, freeze it and make screendumps to compare afterwards.