I expect Starcraft 2 sometime 2009 and think my 2600 hd pro iMac should run it. Diablo 3 however will not appear before 2010 or later, but I'll replace my trusty PowerBook G4 with a mbp by then.
Somewhere around here I believe rbarris (who works at blizzard and hope he forgives if I misspelled his name) posted that they were testing Starcraft 2 on all intel Macs, so minimum requirements for this game could even be ati 1600 at 128 mb. Blizzard is usually quite good at making requirements as low as possible.
This is all personal opinion and guessing of course.
Blizzard games are never graphical powerhouses, at the graphics Starcraft 2 is running now I think even the 7300GT Mac Pro will be able to play it (just not at the highest settings).
Same for Diablo but since that game is probably 2-4 years away the graphics will likely evolve a lot.
Blizzard optimizes their games for the lowest computer specifications as possible. Look at WoW, I can play it on a Pentium II/III. I wouldn't be too worried if your computer has a dedicated GPU, other than the current generation of MacBooks.