I roll my own, thus I endorse IBM PC compatibles. I can fix them, upgrade them, pass them down to the kids. Macs, in my opinion, network a bit easier, but that does not justify the cost! They are also less prone, it would appear, to virusus. Macs lose value faster simply because they are more expensive to begin with. A PC starts off cheap and just gets cheaper.
You can build up a very hot box (i.e., P4 3ghz (3.0E processor) 800mhz FSB w/1gb of RAM, dual 80GB HDD, CD-RW, DVD, wireless keyboard and mouse, nice LCD display) for between $900 and $1,100 depending on what you buy and where you buy it. I do not believe that you could even come close to that kind of firepower on a Mac for that price, or close to it.
You make the call. You say you already have software. Do you want to buy more? Do you have the time and the motivation to do this? Do you have the cash to drop on a dual processor Mac and large format LCD and never miss it? There are a lot of religious viewpoints hereabouts, but the bottom line is that the manufacturers are pretty hot about pursuing the PC market, and somewhat less hot about pursuing the Mac market.
Read into that what you will.