Test Parameters:  Athlon XP2100, 1.5 GB DDR2100 ram, RedHat Kernel 2.4.18-14
Database size 2.9 GB, 1 million rows, PG 7.3

Test 1: 250000 Shared Buffers
db=# explain analyze select count(id) from table;
                                                             QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=100073652.06..100073652.06 rows=1 width=13) (actual
time=2068.12..2068.12 rows=1 loops=1)
   ->  Seq Scan on table  (cost=100000000.00..100070808.85 rows=1137285 width=13)
(actual time=0.02..1447.04 rows=1032956 loops=1)
 Total runtime: 2068.19 msec
(3 rows)

Test 2: 4096 Shared Buffers
db=# explain analyze select count(id) from table;
                                                             QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=100073652.06..100073652.06 rows=1 width=13) (actual
time=3317.00..3317.00 rows=1 loops=1)
   ->  Seq Scan on table  (cost=100000000.00..100070808.85 rows=1137285 width=13)
(actual time=0.08..2775.54 rows=1032956 loops=1)
 Total runtime: 3317.07 msec
(3 rows)