The Democrats need to gain six seats to take over the senate in November. Real Clear Politics has them picking up five. The averages of the major polls on the senate races can be viewed here.
Even if the Democrats don't gain control of the senate an increase of five seats would be very worrisome for the Bush agenda. Throw in the half dozen or so liberal Republicans and the Democrats could have enough votes to block a Supreme Court, or federal judgeship nomination, or almost any piece of legislation that Bush tries to get passed.
For those who are hoping that Bush gets to make another nomination to the Supreme Court the possibility of a 50-50 senate is not comforting.