I trade on a 2010 17" Macbook pro with 8GB of Ram and a 512 ssd (aftermarket upgrade) hard drive.
I have windows 7 in bootcamp as well as parallels. I typically boot my Mac right in windows if the only thing that I am doing is trading. If want to have any other applications open, I run parallels. It runs flawlessly with 8GB of RAM, however, I have a pre-retina display mac. From what i understand, the retina display requires more system resources. If you are going to trade via windows on bootcamp, then the 8GB should be fine. If you intend to get parallels of vmware fusion, then upgrade to 16gb of Ram.
Overall, i love my mac and will never go back to a windows machine. However, if i were to ever set up a desktop for trading only, it wouldn't make sense to get a Mac.
If you intend to trade on your mac, you need to think about putting windows on it. Trading platforms for mac are very limited. TOS is way too expensive if you are going to trade futures.