... but most S.T.A.L.K.E.R. games were incredibly bug-ridden on release. And a pain in the ass to spell.
Taking a look at the Steam Hardware Survey, I can tell you pretty exactly how large a market share would be excluded from Just Cause 2:
15 percent. FIVETEEN.
There's a category called "DX10 Systems (Vista/Win7 and DX10 GPU)" that also contains a percentage for "DX10 GPU on XP", and that number's 14,35%. Now, taking into account some statistics stuff, I'd say this number might have a tolerance of +/- 5 points, so the user share we're talking about is between 10 and 20 percent.
You might also take into account the 27% of users with DX9 SM 2b + 3 cards, but given that the system requirements list the geforce 8800 and radeon 2600 (both DX10 cards!) as minimum, I think we can safely assume that those cards wouldn't be a great idea for this game anyhow (or how many DX9-only cards faster than the 8800 or 2600 are there?).