I would first find out what software you will be using and determine the minmum requirements for same. You may find that your current MBP wil do the job. I use EYE TV which one can record and edit 1080 high definition TV. The demands on a computer will be comparable to film editng and EYE TV will operate with any Core 2 Duo machine. A faster processor will enable to perform certain functions much quicker. When I run my edit compiler, my newer MBP will perform the task in about 40% of the time as my older MBP (I ran an apples to apples test).
My advice is to keep the current MBP if it meets the software requirements. If you find that it's too slow, then purchase a new one. If the software requires more horsepower, then you have no choice but to get a new MBP.
Ciao.