In OS X, a serial port is a serial port is a serial port. PCI, "native", USB, whatever.
On my "Hack Pro", OS X recognizes the built-in serial ports on my motherboard just fine as cu.serial1 and cu.serial2. Likewise, a USB serial adapter shows up just fine as well. Since RS-232 serial tops out at 1/10th the speed of USB 1.1's SLOW speed, there is no reason to insist on a PCI card rather than a USB adapter.
Also, check your motherboard to see if you have an onboard serial header. While it may not have a serial port on the back panel, it may have a 9-pin header, so all you would need is a simple $5 internal adapter.
On my "Hack Pro", OS X recognizes the built-in serial ports on my motherboard just fine as cu.serial1 and cu.serial2. Likewise, a USB serial adapter shows up just fine as well. Since RS-232 serial tops out at 1/10th the speed of USB 1.1's SLOW speed, there is no reason to insist on a PCI card rather than a USB adapter.
Also, check your motherboard to see if you have an onboard serial header. While it may not have a serial port on the back panel, it may have a 9-pin header, so all you would need is a simple $5 internal adapter.