The Saboteur's PC version had big problems out of the box - something like 9 out of 10 ATI-equipped systems couldn't make it work properly. The remnants of Pandemic have rolled a patch, but be sure to read it carefully.
While the 13.8MB patch should fix the ATI issues, Electronic Arts wants you to accept an EULA before you download it. And then, users with quad core CPUs will need to implement a workaround (described here, see "Known Issues & Current Workaround), otherwise they "will possibly have significant streaming issues." This is funny because the game's specs recommend a 2.8GHz Quad Core CPU.
EA says it's aware of the issue and working on a fix for the next patch update. And "while this will decrease the amount of streaming glitches, it is not a fool-proof fix."