Insert Increment Variable

This is on SQL 2000 and 2005. I need to increment a variable on insert, but am having a hard time getting a workable solution:

declare @var int
set @var = (select max(id) from table1)

insert into table1

select val1,

@var +1 as val2
from table2
where blah and blah and blah

If I were trying to insert val1's of (5, 10, 20) with @var having an initial value of 100, the added rows should be:

val1 val2
5 101
10 102
20 103

Does anyone know how to do this without temp tables (or table variables)?