you can't use that modem without an ISA slot.
I don't think its correct to say that PCI modems would be incompatible with your ISP. True, most PCI modems are piece of junk winmodems that have serious issues.
But not all PCI modems are winmodems. Go to
Pricewatch and do a search for "PCI hardware modem" and you'll see a big list of affordable non-winmodem PCI modems.
You might look at external modems. I saw a US Robotics Courier external serial modem at Walmart for $100 the other day. That is an excellent modem. Then again there are cheaper ones out there and even USB modems, though I am not too familiar with them.