Question about shared_buffers and cpu usage

Hi

I am using Postgres8.3 on 8G memory , Xeon X5355 Quad Core x 2
processer RH5 machine with 10G data. (with  some table which have
about 2,000,000~ 5,000,000 rows )

I have two quesion.
1. how to set the shared_buffers and other postgresql.conf parameter
for best performance?
I only run the Postgres8.3 on the machine so I set the shared_buffers
= 7168MB (7G)
But somebody said it is too big, so confused.
The memory info is that
-----------------------------------------------------
MemTotal:      8177484 kB
MemFree:        313336 kB
Buffers:        112700 kB
Cached:        7008160 kB
SwapCached:     210832 kB
Active:        7303660 kB
Inactive:       402088 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:      8177484 kB
LowFree:        313336 kB
SwapTotal:     8385920 kB
SwapFree:      7415768 kB
Dirty:             908 kB
Writeback:           0 kB
AnonPages:       28312 kB
Mapped:        2163912 kB
Slab:            99396 kB
PageTables:      13004 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:  12474660 kB
Committed_AS:  8169440 kB
VmallocTotal: 34359738367 kB
VmallocUsed:    267136 kB
VmallocChunk: 34359470587 kB
HugePages_Total:     0
HugePages_Free:      0
HugePages_Rsvd:      0
Hugepagesize:     2048 kB
-----------------------------------------------

2  I have 8 core cpu ,but It seems that one sql can only use 1 core.
Can I use more core to execute one sql to optimize the speed ?

Thanks