TRD is a based on an equation. Its a function of the FSB and the strap beeing used. In most cases it seems a very high FSB does not help performance. If you have a CPU with unlocked multi, it seems 400FSB is the best setting in most cases, as you get to use the best straps at TRD5.
An overview of what TRD's are available at each strap under various FSB speeds here:
(also suggest reading the entire article)
http://www.anandtech.com/mb/showdoc.aspx?i=3208&p=9
Remember that the auto-setting for TRD is still somewhat buggy on these boards. So set it manually according to the list above. Also, remember that the settings described here are the maximum allowed settings. You need to have memorysticks that are able to handle these, aswell as alot of voltage to the MCH. I recommend a setting of +0.275 to the MCH to reach these and be stable.
TRD Phase is used to set a different TRD on each memory channel. For example, if youre at an FSB that makes TRD6 unstable on both sticks, you can run one on TRD6 and the other on TRD7. There is a table somewhere on the web that explains each of the 31 phase-settings gigabyte uses for this. Asus enthusiast boards have far better bios options for TRD-phase tweaking in my opinion. The Gigabyte one is rather cryptic and strange.
F8c is interesting. I hope theyve done a better job of implementing the 266 2.5A strap, as it should be very useful if working correctly.