Hi,
For most things, you just need a regular DC. The DC has all information on
all objects in the domain. The GC has partial information on objects in all
domains. Universal groups can have members from any domain, so the client
must contact a GC to find all group memberships. Unfortunately, even if you
have one domain, or no Univeral groups, the GC must be contacted to discover
this.
Because the GC has a partial replica of every object in the forest, it can
be large. It requires more hard drive and more replication traffic. Thus, a
trade off is involved when deciding how many to have. The official advice is
to make the first 2 DC's in each domain GC's, then 1 out of every 2
thereafter. Most traffic is between the client and a DC (whether it has a GC
or not).