The server runs well but I wonder if I should get more RAM. I only have a few MB of "free" memory and 1.2GB of "cached" memory:

free:

             total       used       free     shared    buffers     cached
Mem:          3945       3893         51          0         28       1216
-/+ buffers/cache:       2648       1296
Swap:         3895        857       3038

I learned that cached memory is used while it's free and not.

Is the cached value an indicator for the need of more RAM?

cat /proc/meminfo 1 day after flushing the cache:

MemTotal:      4040048 kB
MemFree:         32844 kB
Buffers:         18956 kB
Cached:        1249092 kB
SwapCached:     161576 kB
Active:        3611328 kB
Inactive:       189104 kB
SwapTotal:     3989496 kB
SwapFree:      2894200 kB
Dirty:           20520 kB
Writeback:           0 kB
AnonPages:     2523496 kB
Mapped:         217744 kB
Slab:            70940 kB
SReclaimable:    36756 kB
SUnreclaim:      34184 kB
PageTables:      99648 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   6009520 kB
Committed_AS:  6401716 kB
VmallocTotal: 34359738367 kB
VmallocUsed:     18852 kB
VmallocChunk: 34359719439 kB
HugePages_Total:     0
HugePages_Free:      0
HugePages_Rsvd:      0
HugePages_Surp:      0
Hugepagesize:     2048 kB

top:

top - 17:20:10 up 112 days,  3:06,  1 user,  load average: 1.01, 1.62, 1.48
Tasks: 208 total,   1 running, 207 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.6%us,  0.6%sy,  0.0%ni, 97.5%id,  1.3%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:   4040048k total,  3953108k used,    86940k free,    16348k buffers
Swap:  3989496k total,  1095712k used,  2893784k free,  1235436k cached