realistically speaking any HD2XXX from Ati or 9XXX from nVidia would do most of what you'd need for a HTPC. For Games, you'll basically just need to spend north of $50 to get something that will work, but not spectacularly, $100 and up should play anything out now well enough for anyone but the "pro's." Most of the highest end stuff is relegated to bragging rights.
The only limitations you'll run into is that none of them will bitstream 8 channel audio, but the ATI HD46xx and DH48XX can do PCM 8 channel audio, which just means that the audio is decoded by software and the individual channels of sound are sent across HDMI instead of the encoded audio stream. There really isn't all that big a difference.
I personally prefer the nVidia drives by a long shot, so I'll be sticking to their stuff for now, even though the ATI hardware is a better value, their drivers are frustrating and take up a bit more space than the nVidia ones, IMHO. My next is likely to be a 9800GT from XFX.
It depends on what you want. Start with what htpc duties do you want it to pull. Is audio over hdmi important to you? If yes, then ati 4xxx cards are the way to go. picture quality wise, unless you are an uber geek (not saying this as a bad thing... a compliment actually) either nvidia or ati cards are fine and damned near anything modern can play hd files just fine. as far as gaming goes, ATI cards are a better value. If you are on a budget, but still want to game into the near future at respectable settings, look into the radeon 4770. It is getting glowing reviews just about everywhere and only costs about $100. a 4670 will get you by just fine, but i think it is worthwhile to go the extra mile and get something that will last a little longer. 4770 or a 4850 if you can find a great deal that makes it significantly cheaper than a the 4770.
beyond that, the way to pick a card is to haunt the hardware review sites. my favorite is anandtech.com. they give some of the most honest and in depth reviews of hardware on the net. they take it to a detail level that is beyond many of us here, but manage to write it in a way that actually teaches you something. After you've done the research, you have to just weigh cost vs. benefit, just like anything else...
I've been using catalyst 9.7 drivers for my ati card in win7 x64 rtm and everything has been gravy so far... Can't comment on nvidia drivers. I have a nvidia mobile graphics chip in my laptop running win7 x86 rc1 and that hasn't given me any problems at all, but then again, it usually gets used only for database work, browsing, and occasionally a game of plants vs. zombies or civ4 if i'm on the road. I'm pretty sure nvidia doesn't want to repeat what happened when vista was released, and has put forth the extra effort with win7.
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