The OEM partition enables Dell utilities which may or may not work when you clean reinstall. Most of these utilities have better options built into Windows 7.
The REcovery partition works quite well in Dell to restore to factory condition from boot, which you might want to do if you should decide to sell the computer but keep Windows 7 for yourself. Hence it would be wise to at least make the Recovery disks, although they can always be ordered from Dell, or a Vista installer for your version can be located to clean reinstall your version with Product Key on COA.
With these things in mind, decide if you want to delete those partitions. If so the best method is to wipe the HD using the tutorial BarefootKid wrote and linked. This gives you the cleanest possible slate for new install: SSD / HDD : Optimize for Windows Reinstallation