You should see a slight benefit with the 8-core in FCP but to get the full benefit, you'd have to setup clustering. It's like turning your machine into a mini render-farm so it splits the job into chunks and processes them separately then joins them up again:http://www.barefeats.com/octopro5.html
I doubt Aperture would show any benefit with 8-cores until you do batch operations. Logic would if you process lots of tracks but the quad cores have hyper-threading anyway so probably only with more than 8 tracks.
Some tasks will be limited by your hard drive too like say you have a 90 minute 1080p ProRes file in FCP that takes up 200GB. Regardless of processor speed, if you were to encode that file, it has to read 200GB off your disk, which would likely be only 80MB/s so it'll still take 40 minutes to encode even if you had a 20THz processor.
It boils down to how you use software and not the software itself. If you own Logic and only mix recordings of your karaoke in the bath with pop music, you don't need 8 cores. If you own FCP and use it to edit your home movie from a DV camera, you don't need 8 cores. If you have Aperture and you use it to correct red-eye in pics from the office party, you don't need 8 cores.
Generally people who need 8 cores don't need to ask so if you need to ask, chances are you won't benefit from having 8 cores. But, the upgrade cost is only $775 so if you have nothing better to spend that money on right now, there's no harm in future-proofing your machine a bit and software improvements are always trying to find ways to utilise more cores.
If that's your budget and you have a low quality screen, I'd get the quad and get a decent IPS display and some extras like Applecare and some neat peripherals like the magic trackpad and so on.
When you make an investment in a machine like the Mac Pro, it's not an expense as it has resale value. So even if you decided after a year that it struggled with what you do, you can sell it on and only pay the upgrade to a faster one.