Here is a nice article series posted by Ricket Minerich on his blog.

The Garbage Collector is one of those things that I tend to get very passionate about (I guess it must just be esoteric internals or something).  I thought Rick did a very good job explaining the various modes you can use with the GC. 

Changing your GC settings on the fly

Of course I have to add that you almost never should adjust these settings.  Only in cases where you have spent extreme amounts of time validating these decisions should you do this type of thing.  Seriously!  Your changes can impact the performance of the entire machine (not just your process).