I've considered adding a PC, but most of my work, like yours, is also Mac-only. But I'd need a lot more justification for mastering a different OS than LCC/eLC.
On that note, I'm seeing Core 2 Duo Quads for $2300 now, which is a lot less than what I paid for my 2 x Dual-Core. Getting a refurb 2009 model might be a consideration for me, but it's always a toss between getting the "latest and greatest" for the sake of "longer-gevity" or playing it safe with an older system with the risk of a shorter shelf life.
It's not clear when Apple will release the next wave of MacPros, but some have hinted at Fall, 2010. One thing I find encouraging comes directly from Vienna Ensemble. Not having to buy audio and midi interfaces means that those funds could be directed (in whole or in part) towards a newer machine as new technology would be aimed to support them. When I bought my 2 x Dual Core a couple of years ago, I had NO IDEA that it would not boot in 64-bit mode once Apple got around to releasing an OS with at 64-bit kernel. I was told that it was "64-bit-ready". At the time, I suppose it was since Apple was still considering support of 64-bit Carbon frameworks.
But the rug was pulled from beneath many of us. My 2 x Dual Core, as a result, simply became the machine that my older G5 hoped to be instead of being the viable investment in a true 64-bit future that I thought would happen by now on the Mac platform. It's close, but no cigar. This alone remains a viable temptation for building at least one PC monster sidecar.
Much to consider.