I have used all Mac trading set up for years. If you can work with available software, its the way to go IMO. There are no viruses, worms and almost no spyware and adware. Plus as a UNIX system it is virtually uncrashable and you will have up-times of months.
You can hook up one external monitor to recent Powerbooks- they have a dvi port.
I have used IB and Thinkorswim for brokerages and Linnsoft with DTNIQ as data feed- all work natively and well in OS X. In terms of Virtual PC, I used to use it to run IB before it went java. But IB was a pretty light CPU using app and this might have been OS 9 even so my experience is not up to date.
In terms or running a real time trading app through Virtual PC, I have not tried this yet. However I use Virtual PC and am pretty sure it would be fast enough on a modernish dual G4. I have a dual 1.25 G4 and it runs fairly quick on it. Virtual PC does not yet work on the G5s. Microsoft has purchased the product and is supposed to come out with a much improved version later this year that runs on the G5s.
I am likely going to have to soon use some PC only software and am dreading the idea of buying a PC. I will try it first in Virtual PC for no other reason than knowing whatever hell breaks loose on it, its just one file that can be trashed and restored from backup....
Steve, as of yet there is no native mac software to use Esignal though I have seen a rumor that the developer of ProTAgold,
http://beesoft.net is working on a real time version that uses esignal. Otherwise the 2 main choices for mac are softwares that run DTNIQ or MyTrack.
As you mentioned, you will be free of most of the security headaches from the wintel side. OS X has a built in firewall but I would still run an external router firewall, just for added insolation and stealth surfing.