You should use the PCI card, it's a much better quality card. Not that it would limit your Internet connection speed though, the cable modem won't be even close to as fast as that old ISA NIC. However if you ever set up a LAN at home, the ISA card would be quite limiting.
There are two things you should be aware of before you do this. First, many broadband connections limit you to connecting to a single network card. Sometimes they hard-code the MAC address of your network card into a system on their end to allow you to connect. Most systems have been upgraded and don't require this anymore, but you never know. You want to take a look at the ISA NIC for a sticker on the back and make note of the MAC or hardware address there. It's a hex address that should look something like:
00-10-B5-BC-DD-66
Your newer PCI card will very likely have the capability to report this number if you want, so you wouldn't have to use the ISA card. You simply set up your PCI card in Windows to pretend to be that MAC address, and it should work fine. If it's a newer system, you may not even have to do that. Write the number down just in case, but try the PCI card anyway because it may work just find in a modern cable modem system.
The second thing you need to think about is what IRQ's are available on your machine, and what PCI slots are assigned to those IRQ's. In today's complex integrated mainboards, some slots normally share IRQ's with other devices, while some PCI slots have their own IRQ. You want to avoid sharing IRQ's with anything if possible, but especially your video or sound cards. Check your mainboard manual to find out what slots share IRQ's and what slots have their own IRQ and put the network card in one that has it's own IRQ. Generally you want to avoid the first slot under the AGP slot, and sometimes the last slot as well. Often one of the other slots shares IRQ's with the USB bus, so check your mainboard manual first to avoid problems.
