There is plenty of material available to help you upgrade to SQL Server 2008. This blog is intended as a short list for the most useful guidance that I have found. And you may have different experiences upgrading from SQL Server 2000 and SQL Server 2005 so please take the time to do proper preparation work and advanced studying. And test your upgrades before you do it for real in production.
There is one bug that you need to know about that potentially affects your upgrade experience. If you are planning on using backup/restore to move your database over to your new SQL Server 2008 environment, then you will need to start with the latest Cumulative Update. Get that installed before you do the restore as it contains a fix you will need. The problem is that the log replay after the restore may take a very long time, up to 10x the time you expected. It only happened when log files were very large and had many VLFs (virtual log files). Detach/Attach method is not affected. This is the KB article for the error: http://support.microsoft.com/kb/967178/EN-US/. The fix is in CU4, but you should get the latest one available (as of the time of this writing it is CU4).
Here are the minimum steps I recommend:
Start with the SQL Server 2008 upgrade advisor: http://www.microsoft.com/downloads/details.aspx?familyid=F5A6C5E9-4CD9-4E42-A21C-7291E7F0F852&displaylang=en. Read all output in detail because it can be easy to miss a small warning that could be important.
Then get the 490 page document available on the web that contains all the upgrade best practices. http://www.microsoft.com/downloads/details.aspx?FamilyID=66d3e6f5-6902-4fdd-af75-9975aea5bea7&displaylang=en. It has a good section on Windows failover clustering.
Another great site is http://msdn.microsoft.com/en-us/library/cc936623.aspx. Although there is some overlap with the 490 page document, it contains more detail for some sections, especially clustering. This site is an absolute requirement to use if you have Windows failover clustering.
See my other blog on this site that I co-wrote with Glenn Berry on how to use database Mirroring to minimize downtime if you are upgrading from SQL Server 2005. http://blogs.msdn.com/sqlcat/archive/2009/02/09/minimize-downtime-with-db-mirroring.aspx
Motivation to upgrade if you are using failover clustering: http://sqlcat.com/top10lists/archive/2008/11/20/six-failover-clustering-benefits-realized-from-migrating-to-sql-server-2008.aspx