Linux is using some memory for file caching, but it is only a small part of all the memory overhead that increases during the day (and during the night... always, little by little, but without stop). Here is the result of the top comand:
top - 14:07:54 up 1 day, 1:34, 1 user, load average: 0.07, 0.03, 0.01
Tasks: 67 total, 1 running, 66 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0% us, 0.0% sy, 0.0% ni, 100.0% id, 0.0% wa, 0.0% hi, 0.0% si
Mem: 2056168k total, 675732k used, 1380436k free, 99816k buffers
Swap: 4192924k total, 0k used, 4192924k free, 187620k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
[...]
6088 mysql 16 0 684m 117m 3928 S 0 5.8 0:00.24 mysqld
5869 apache 16 0 200m 38m 29m S 0 1.9 0:00.45 httpd
5873 apache 16 0 201m 25m 16m S 0 1.3 0:00.37 httpd
5872 apache 17 0 201m 25m 16m S 0 1.3 0:00.25 httpd
5868 apache 17 0 199m 24m 17m S 0 1.2 0:00.38 httpd
5870 apache 16 0 200m 24m 16m S 0 1.2 0:00.26 httpd
5874 apache 16 0 200m 22m 13m S 0 1.1 0:00.48 httpd
5877 apache 17 0 198m 17m 10m S 0 0.9 0:00.12 httpd
5871 apache 17 0 198m 16m 9928 S 0 0.8 0:00.13 httpd
5876 apache 17 0 198m 15m 9060 S 0 0.8 0:00.18 httpd
5875 apache 17 0 198m 15m 9072 S 0 0.8 0:00.15 httpd
[...]
Brainstorming needed