While there is some excellent ham radio software for the Mac such as CocoaModem, Aether, EchoMac, and DSP Radio, there's a lot of good programs which are unfortunately only available for Windows. One of my favourites is CwGet which seems to be the best Morse decoder around.
In the past I have tried Parallels but have always had problems getting sound in and out of the Windows virtual machine.
I've just upgraded to Parallels 4 and, apart from being generally smoother, simpler and faster; connecting a USB device such as the SignaLink USB I use for radio interface seems to work really well. Here's the preferences I use:
It's no surprise that the two operating systems compete for the USB audio device. I had CocoaModem running and then started XP under Parallels and CocoaModem's waterfall froze. The program was able to quit cleanly.
Anyhow, this is all much more convenient than a separate machine just to run Windows ham radio software.
Parallels 4: recommended.
Update - transmit audio trouble?
Haven't quite diagnosed this yet but I ran WSPR under Parallels and while I could spot other stations, no one spotted me. I listened to the off air audio and it didn't sound as pure as normal.
I take that back, it turned out that a station that should have received me had a problem. The run over night shows that wspr runs fine under parallels.
But the best news was that I spotted old friend Ross, T61AA, in Afghanistan multiple times over night. (He's the person that got me started with WSPR).