Supposedly, nVidia cards from the FX5000 series and newer (which should include all PCIe cards) can go down to 15kHz, but I've not had reason to try this since my monitor can do 800x600 (actually, it'll go up to 1080i).
However, Blitz cabs use medium res monitors, so if it still has the original monitor in it, you can probably get standard progressive 640x480@60Hz out of it. Check the specs on the monitor to be sure (30-31.5kHz horizontal, 60Hz vertical). Note that XP won't let you select this from the normal GUI: you'll have to futz around under advanced, where all the useful options are. My solution is to just run Linux and use a modeline to guarantee the exact timings I desire.
Also, VGA text mode is at 70Hz (and I've also seen 72), not 60, and most single mode arcade monitors won't take kindly to that, so beware of that, too. The solution to this seems to be doing a divide by two (or even a full-out interlace using a line buffer) when 60Hz, 480 line progressive material (such as standard 640x480) is being displayed, and simply shutting off the sync when anything other than 15/30kHz horizontal and 60Hz vertical is being displayed to prevent damage to the monitor.