Before doing anything else, make sure you have a complete backup of your OS X partition you can get back to in case of an unexpected problem.
Once you're ready, the quickest thing to do is go back into Disk Utility and reformat the lower (I presume empty partition) as MS-DOS, which will make it a FAT32 drive; the same thing the Boot Camp Assistant would do. Give it an obviously recognizable name such as "Win7".
Mind you, this is the manual method I'm describing. Next, put the Win 7 disk in the drive. Open the System Preferences, click on the Startup Disk icon and choose the Win 7 DVD. Restart.
Depending on what Mac model you have, you may not be able to use the 64 bit version of Win 7. Your Mac has to support a 64 bit boot sequence for that to work. If nothing else, you'll know if the Mac can't boot to the 64 bit DVD. Use the 32 bit DVD if that's the case.
Once the Mac boots to the DVD, the first thing you must do is reformat the FAT32 drive as NTFS. Obviously, choose the drive you reformatted as FAT32. The Win 7 setup will recognize the name of the FAT32 drive. The Mac partition will be labeled unknown, so it's easy to tell which is which. Finish installing Win 7.
The more automated way:
Go into Disk Utility and delete the lower partition. Drag the upper partition down to fill the freed space. Click Apply.
Launch the Boot Camp Assistant. When it displays the graphic of partitions, make sure to slide the horizontal line to make the new partition for Win 7 bigger. The assistant defaults to a measly 32 GB partition. Hardly enough to hold Windows, much less the other apps I imagine you'll install.
When the partitions are to your liking, continue on booting to the Win 7 DVD. From here it's the same as above. First reformat the new partition, which will be named BOOTCAMP, as NTFS. Install Win 7.
When Windows is done installing, it won't look like much because no drivers for the Mac hardware have been installed yet. From the Windows desktop, put the Snow Leopard disk in the drive. From the menu choices that pop up, tell Windows to Run Setup.
That gets the initial drivers installed. You'll now have a choice in the Start menu for Apple Software Update. Run that to get and install any other Boot Camp driver updates.
