There are numerous benefits to running a machine with either a dual core processor or one that has two physically separate symmetric CPUs.
The main benefit is big speed increases in applications written in multi-threaded form. Adobe Photoshop is one of the most popular multiprocessor enabled programs (it has been SMP-enabled on the WinNT side for a decade -well ahead of the Mac version). Every professional level 3D app (Maya, Lightwave, Softimage, Autodesk Studio3D Max, etc.) is also SMP-enabled. Numerous video and motion graphics editors, such as Adobe After Effects, Premiere Pro, Combustion, Apple Shake, etc. will take advantage of multiple processors as well.
None of the vector drawing applications and traditional sign making applications are SMP-enabled. I don't know for sure about some high end RIPs, such as Onyx PosterShop Pro.
Even if your application isn't SMP enabled, Windows2000 Professional, WinXP Pro and MacOS X will assign any open applications across the two or more processors or cores. You'll be able to do certain things more safely, such as burn a DVD in the background while doing some other stuff in a different application. The ability to copy large files from one folder to another without the operation commandering your machine is improved.
Chip makers have hit the limit with how fast clock speed can be with a single core. The cancellation of the 4GHz Pentium IV underscored that. Intel and AMD both have planning roadmaps that clearly show dual core and multi-core taking over for the rest of this decade. Around 2008 Intel plans to release one high end desktop processor with 8 cores! By 2010 you probably won't be able to buy a computer with any less than 2 or 4 cores.