GiST memory usage

Hi all.
I've been reported memory limit hits
during a postgis index construction
(GiST index).

The indexed tuples are about 425 millions.
The index key is a box2d object (4*sizeof(float)).

The machine has 4 Gb of RAM, and operations are
peaked at ~2.4 Gb of usage.

Is there a way to detect where is memory hold ?

Is there an index-specific memory context for use
to early release memory blocks allocated during
operations (didn't see any use of memory contexts
in the contrib/rtree_gist) ?

TIA

--strk;