Hi GG,
I am currently doing all my trading through a linux box, work as a linux systems admin for University of Melbourne (and previously same job at CSIRO in Sydney) and have been using linux since the age of 15.
IB TWS works fine in Linux/Mac as it runs through java. I understand NinjaTrader runs fine through java on the mac also. Haven't used webIRESS but assuming the same.
There is a commercial version of wine called Crossover which costs <100USD and I use it to run apps like Metatrader4 and some games. It is specifically designed for MS Office and Photoshop users who want to run in Linux. Aside from a few extra font and DLL requirements MT4 works great. I am sure the Mac version of crossover is what you want.
For everything else there is VMware (or its mac specific competitor, Parallels).
Any questions, feel free to ask. My advice is that if you have a $10,000 budget, you might as well buy all the Mac stuff you want as well as an $800 Dell plus a KVM switcher for trading.