geometry poligons performance and index

Hi
I need to set a query which performs a intersection check between two set of
polygons which are stored into two tables.
I wrote the where instruction so:
...
where area(intersection(t1.the_geom,t2.the_geom))>0
...

The results are ok, but the query is too slow. I tried to applay a GIST and
gtree index but performance did not improve.
By explaining the query I see that the where condition gets the query slow.
Is there a more simple way to check if two poligons have a not null
intersection (they overlap)?

Is there a kind of index I can use ?

THanks

Giu 
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