Postgres not using array

Folks,

In a fresh Pg8.3.3 box, we created a 3-stripe RAID0 array.
The write speed is about 180MB/s, as shown by dd :

# dd if=/dev/zero of=/postgres/base/teste2 bs=1024 count=5000000
5000000+0 records in
5000000+0 records out
5120000000 bytes (5,1 GB) copied, 28,035 seconds, 183 MB/s

BTW, /postgres is $PGDATA ...

Unfortunely, we cant figure out why Postgres is not using the disks at all.
Right now, there are 3 heavy queryes running, and the disks are almost
sleeping...
The CPU is 100% used since a few hours ago. Can anyone tell why?

# dstat
----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq|_read _writ|_recv _send|__in_ _out_|_int_ _csw_
 99   1   0   0   0   0|   0     0 | 574B 1710B|   0     0 |   6    55
 99   1   0   0   0   0|   0     0 | 290B  770B|   0     0 |   8    48
 99   1   0   0   0   0|   0     0 | 220B  540B|   0     0 |   6    44
 99   1   0   0   0   0|   0     0 | 354B  818B|   0     0 |  10    50
 98   2   0   0   0   0|   0     0 |1894B 4857B|   0     0 |  27    78
100   0   0   0   0   1|   0     0 | 348B 1096B|   0     0 |  12    45

The specs:
Intel(R) Pentium(R) Dual  CPU  E2160  @ 1.80GHz
Linux dbserver4 2.6.24-etchnhalf.1-686-bigmem #1 SMP Mon Jul 21 11:59:12
UTC 2008 i686 GNU/Linux
4GB RAM

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