(no points ...)
I can only tell you a few basics about MS SQL... and maybe recommend you split the question up a little differently so you get a better answer. The reasons being a) this is more of a db zone question than CF and b) not every db expert knows *both* MS SQL and MySQL. So IMHO, you may want to split this into 2 questions: 1 for the MS SQL portion and 1 for the MySQL portion.
The MS SQL 2000 equivalent of varchar(max) is the "text" data type (or ntext for unicode). The maximum capacity is 2,147,483,647 for text and 1,073,741,823 for ntext
http://msdn.microsoft.com/en-us/library/ms187993.aspxYou CAN'T index "text" fields in MS SQL. But you can set up full text searching, which is whole 'nother topic in and of itself ...
http://www.databasejournal.com/features/mssql/article.php/1438211/Setting-Up-Full-Text-Search-A-Step-by-step-Guide.htmIf db storage is not a requirement, there's obviously other search mechanisms available like Verity (CF8) or Solr (CF9).
Good Luck!