Obviously there is no exact answer to your question.
However, I will tell you I have a quad core 2007 3ghz mac pro with 11GB of RAM and I find it too slow sometimes. I have very little patience with computers - I want my creativity to flow freely, not wait on the machine to do something.
If you're doing it for a living, time is money - so get the fastest you can afford. There are a lot of people running LR and PS on iMac so it certainly can be done, but especially given that GPU speeds are limited I would never use one for my primary editing machine and certainly not a laptop.
I use my machine 90% for photo editing, most commonly in aperture and CS3, 10MP RAW files from a 1DMIII
It sounds to me like you are throwing money into the wrong parts of your computer. 4x3ghz w/11GB of RAM is fast enough for just about anything being done right, let alone still photography. That's the sort of rig they edit HD video on, in near realtime.
I'm betting it's your HDD setup that's slowing you down. You should hook a RAID5 made up with 10k RPM drives to that and see how it works for you. I'm guessing you are having load times between opening/saving/changing files. Even 10MP RAW files are only, what, 10-12mb each? You can cache hundreds of those in RAM with 11gb of memory, so it must be harddrive load times you are "feeling".
A dual core MacPro with 4gb of RAM and a really fast HDD setup would likely feel faster than a quad core with three times the RAM editing photos. I'm not a photographer, but I am a network/system admin/tech and have experience working with huge amounts of data.