depends on what you mean by play.
D3 is surprising light on the system. most $150+ cards today will be able to max its gpu settings. But the game hasn't come out yet, and everyone knows that Blizz games are patched with better graphics within a year or so of release. With that said, I played the beta on my GTX 280 on max settings without an issue.
SC2 on the other hand is surprisingly demanding to keep a high average fps @ maxed settings. First of all, your CPU will be your first hurdle. You mention your system has a quad core, but if its not one of the later Intel chips you will not be able to play the game on ultra without some lag, especially in large battles. So regardless which GPU you buy, you will never achieve the brilliant game play in maxed settings if you play competitively. Unless you think playing SC2 on max is not demanding, everyone needs to really see the game in full glory, its a sight to be seen.
My friend and I run similar fps in SC2, and he bought a brand new 6950 about six months ago. My card was released six years ago. Note, we both have i7s. It's only when you combine an i5+ card with the GTX 580 or the HD 7950 or above will you see the next level of FPS jump towards the 100+ fps averaged. That is what I want, but I'm not willing to pay for it just yet.