i5 or i7 processor on 13-inch MacBook Pro

Hey guys and girls!

I'm in need of some expert advice on buying a new MacBook Pro computer. I'm not sure would it make more sense to buy a cheaper i5 model and add extra 4 gigs of ram or to go with i7 with 4gb of ram? I already do have a spare 500 gigs 7200 rpm hard drive.....

In most times, I'm using the computer for regular office tasks. During the weekends some photo and video editing (novice level, using iPhoto, Photoshop, iMovie). Every now and then, some live flash video encoding (live Ustream) is also what i need.

I do need my computer to be fast even the weekends and would like to be able to run for many years. My current black MacBook is about 4 years old and is still doing fine and thats what I love about MacBooks.

Any advice would be lovely!