We also have seen differences between SQLServer 7 and 2000. We looked for the differences with the profiler on both servers (And sqlserver 2000 with 70 compatibility) looking using sql statements, stored procedures , RPC's (used by our centura application and I have heard also by ADO) and locks.

Save the results to text files and use windiff.

If you are curious, in our case there were no statement differences but was SQLServer 2000 with 80 compatibility much slower than SQLServer 7 and SQLServer 2000 with 70 compatibility, resulting in long held locks.