reducing the poll rate increases prediction so dropping from 500 to 250 will increase the prediction by as much as 200% so your mouse will be more likely to snap to angles and straight lines rather than creating smooth curves.
250 is the minimum needed for gamers as a typical reaction time is about 1/4-1/8 of a second. having it set to 500 will mean you will always have the response you require from your mouse... having it set to 1000 isnt generally needed but is added because if your moving left to right the response to the change in direction can be near instantaneous rather than waiting 1/8th of a second which is measurable by the brain the pointer will appear to react as quickly as you do.http://www.ngohq.com/news/15043-how-to-increase-usb-sample-rate-in-windows-vista-7-a.html#post73228
check here if you want to experiment with your mouse...
even a mouse that seems to be set at 125 can be set as high as 1000 by changing how fast windows not the mouse responds. you wont be able to change the dpi but increasing the poll rate will change the overall accuracy of your mouse.