Memory reporting on CentOS Linux

I am confused about what the OS is reporting for memory usage on CentOS 5.3 Linux. Looking at the resident memory size of the processes. Looking at the resident size of all postgres processes, the system should be using around 30Gb of physical ram. I know that it states that it is using a lot of shared memory. My question is how to I determine how much physical RAM postgres is using at any point in time?

This server has 24Gb of ram, and is reporting that 23GB is free for use. See calculation below

(Memory Total - Used) + (Buffers + Cached) = Free Memory
(24675740 - 24105052) + (140312 + 22825616) = 23,536,616 or ~23 Gigabytes

So if my server has 23Gb of ram that is free for use, why is postgres reporting resident sizes of 30GB? Shared memory is reporting the same values, so how is the OS reporting that only 1Gb of RAM is being used?

Help?

top - 12:43:41 up 2 days, 19:04, 2 users, load average: 4.99, 4.81, 4.33
Tasks: 245 total, 4 running, 241 sleeping, 0 stopped, 0 zombie
Cpu(s): 26.0%us, 0.0%sy, 0.0%ni, 73.9%id, 0.1%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 24675740k total, 24105052k used, 570688k free, 140312k buffers
Swap: 2097144k total, 272k used, 2096872k free, 22825616k cached
---------------------
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
19469 postgres 15 0 8324m 7.9g 7.9g S 0.0 33.7 0:54.30 postgres: writer process
29763 postgres 25 0 8329m 4.5g 4.5g R 99.8 19.0 24:53.02 postgres: niadmin database x.x.x.49(51136) UPDATE
29765 postgres 25 0 8329m 4.4g 4.4g R 99.8 18.8 24:42.77 postgres: niadmin database x.x.x.49(51138) UPDATE
31778 postgres 25 0 8329m 4.2g 4.2g R 99.5 17.8 17:56.95 postgres: niadmin database x.x.x.49(51288) UPDATE
31779 postgres 25 0 8329m 4.2g 4.2g R 99.1 17.8 17:59.62 postgres: niadmin database x.x.x.49(51289) UPDATE
31780 postgres 23 0 8329m 4.1g 4.1g R 100.1 17.5 17:52.53 postgres: niadmin database x.x.x.49(51290) UPDATE
19467 postgres 15 0 8320m 160m 160m S 0.0 0.7 0:00.24 /opt/PostgreSQL/8.3/bin/postgres -D /opt/PostgreSQL/8.3/data
19470 postgres 15 0 8324m 2392 1880 S 0.0 0.0 0:01.72 postgres: wal writer process