I'll just give you an indication from personal experience, but I'm not really an expert on motherboards; whenever I need one, I always look through some guides and reviews to make my choice.
About a year ago, I've built an HTPC, and I was also doubting whether to stick with the onboard card or not. In the end, I sticked with the onboard one, and to this date, that has worked beautifully.
The board I got then was for the AMD platform:
K9A2GM-FIH
It can handle any video playback (yes, that includes HD and x264; although I must admit I haven't tried Bluray because..well, we don't care about Bluray ;$), and I've left Aero enabled and don't experience any slowdowns because of that. It also runs Mediaportal just fine, so I'm very happy with my choice.
Now, I'm not entirely sure what the good chipsets would be for an Intel platform (the above is an AMD 780 chipset, which has an ATi Xpress 1250 onboard), but my guess is that something like
P6NGM-FIH would do the trick as well.
Small sidenote: obviously, because I was building an HTPC, HDMI was a priority for me, so that's also what I'm using as output. The board mentioned above is just an example, I have no idea how good the graphics controller on the board really is, but that one has both HDMI and D-Sub ports, so that's the kind of thing you're looking for I guess.