So far, I've been very pleased with my HD 3870 512 MB card. But, ASAP, I'll be getting a 4870.
The only DX10 full title I have is Crysis. I completed it, however, on a HD 2600XT and did OK with it. I've played some of it on the 3870, but, not enough to give a good answer.
The games I DO play, however, I've been very happy with the overall performance.
You may want to ask at which settings the games are being played in order to make them playable, though.
Oh, BTW, here's a link to the current and upcoming DX10 games....
http://en.wikipedia.org/wiki/List_of...ctX_10_support