Many factors will determine transfer speed.
For starters, although the bit rate on your wire is gig, some of those bits are required for "packaging" your actual data. Assuming the transfer is running on top of TCP and assuming your using "normal" Ethernet which allows 1500 byte packets, you'll lose about 3 to 4%. The overhead decreases if jumbo Ethernet is supported, and rises (much) as you move to smaller and smaller packets.
The 3 or 4% is a minor loss, but much more can be loss due to client application protocols (e.g. Windows original CIFS protocol is often slower than FTP), client stack implementations (Windows XP can be much slower than Windows Vista), the quality of the host NICs and their drivers, and also the performance of your gig switch.
At gig speeds, you can also run into issues that the end hosts can not feed or receive at full speed due to disk architecture limitations (PATA, vs. SATA, vs. SCSI; drive specifications; single drive vs. RAID; etc.)
If you can hit about 90% of line rate, you're doing well. If you can't get above 70%, you might need to run down which of the prior issues might be causing the slowness.
Do remember, lots of clients reports speeds in bytes per second, but network speeds are reported in bits second.
