http://support.microsoft.com/kb/923247

Using DBCC CHECKDB (0) WITH ESTIMATEONLY will estimate space needed in tempdb when DBCC CHECKDB is run.
Sample Output:
Estimated TEMPDB space needed for CHECKALLOC (KB)
————————————————-
27

(1 row(s) affected)

Estimated TEMPDB space needed for CHECKTABLES (KB)
————————————————–
678