Since IPv6 will become the main growth protocol on the Internet around 2011, it is a good idea to keep IPv6 enabled and to learn how to do these things right.
The address that you mentioned, 192.168.x.x is an RFC1918 private address in IPv4. It is intended for traffic which is private to one network (or organization) and does not normally cross the organization's network boundaries. RFC 1918 cause people a lot of grief when companies are aquired because people often discover that two or more networks are using the same addresses. In some large companies, they have run out of RFC 1918 address space and have had to supplement it with registered addresses.
In IPv6, the equivalent type of address is called ULA. However, the block set aside for this usage is almost inconcievably vast. There is no way that anyone could ever use it up, and to avoid the possibility of collisions as much as possible, the IETF has asked people to pick their ULA block randomly. The easiest way to do this is to use the generator tool at SixXS. Type in a MAC address from your network card, click generate, and you have a ULA block. You can then use this block to assign IPv6 static addresses and subnets to all devices at your site. You can use any MAC address, it is just there to seed the random number generator.
If you want to, you can also register your use of that ULA prefix at the SixXS page, but you don't have to do this.
Note that the /48 gives you 16 bits of space to break out /64 subnets, so if you are just setting up one server on one network, then pick one /64 subnet and use one address. People often reserve low addresses for various things such as ::1 for a router.
Assuming that you choose the ULA fdec:c0bb:c329::/48, then you could pick fdec:c0bb:c329:0001::7 for your lucky new mail server on fdec:c0bb:c329:0001::/64
