Easiest option:
Replace your old mac with a new mac.
Replace the firewire drive with a USB 3 drive (SSD if you're keen - i've tested a Samsung 850 EVO SSD via USB3 and it gets over 300 megabytes per second which is faster than the internal SSD in my Surface Pro 4 generally manages)
Continue doing the same thing you're doing. No behaviour change or workflow change for you is required.
Windows 7, 8 or 10 will run in a VM just as well as patched XP pretty much, and the new hardware will handle it just fine.
Replace your old mac with a new mac.
Replace the firewire drive with a USB 3 drive (SSD if you're keen - i've tested a Samsung 850 EVO SSD via USB3 and it gets over 300 megabytes per second which is faster than the internal SSD in my Surface Pro 4 generally manages)
Continue doing the same thing you're doing. No behaviour change or workflow change for you is required.
Windows 7, 8 or 10 will run in a VM just as well as patched XP pretty much, and the new hardware will handle it just fine.