With the transfer limitations of mechcanical hard drives today, you will not see 1gbps (1000mbps or 125MB/sec) transfer rate between PCs. To get anything close, you'd need to setup a RAM drive.
As for PCI slots, 33MHz PCI slots have a transfer rate of 133MB/sec. 100mbps network cards will use 12.5MB/sec maximum of that (100mbps/8 = 12.5MB/sec). Gigabit network cards 1000mbps at full speed will nearly saturate a PCI bus (1000mbps/8 = 125MB/sec).
Anyway...I have about five PC's running on gigabit and the best I've ever seen is 20% usage of 1000mbps (1000 * 0.20 = 200mbps) using CAT5e/CAT6 (makes little difference) with a few PCs with a PCI gigabit NIC and other onboard. Another thing is if the onboard gigabit network card is wired directly into a PCI bus, PCI-E bus, or into a special transport similar to what Intel developed for their onboard gigabit NICs.