ho provato a fare quanto detto sacarde, questo è il risultato
codice:
top
top - 09:20:48 up 13 min,  3 users,  load average: 0.16, 0.49, 0.49
Tasks:  84 total,   1 running,  82 sleeping,   0 stopped,   1 zombie
Cpu(s):  6.0% us,  0.7% sy,  0.0% ni, 93.3% id,  0.0% wa,  0.0% hi,  0.0% si
Mem:   1036632k total,   540740k used,   495892k free,    64348k buffers
Swap:  1558264k total,        0k used,  1558264k free,   326528k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 3975 alx       15   0  119m  49m  23m S  0.0  4.9   0:40.07 firefox-bin
 2999 root      15   0  169m  35m 4800 S  2.0  3.5   0:37.13 X
 3903 alx       16   0 47860  22m  16m S  0.0  2.2   0:01.09 zen-updater
 3896 alx       16   0 36636  18m  14m S  0.0  1.8   0:00.51 konqueror
 3828 alx       15   0 34388  17m  14m S  0.3  1.7   0:03.62 kicker
 4099 alx       16   0 32096  17m  13m S  2.0  1.7   0:12.48 ksysguard
 4070 alx       15   0 33344  16m  13m S  1.3  1.6   0:01.56 konsole
 3351 root      34  19 70064  16m 7436 S  0.0  1.6   0:01.52 zmd
 3826 alx       16   0 31904  15m  12m S  0.0  1.5   0:00.51 kdesktop
 3814 alx       15   0 33488  15m  12m S  0.0  1.5   0:00.60 kded
 3893 alx       16   0 31156  15m  12m S  0.0  1.5   0:00.23 kmix
 3891 alx       16   0 31464  13m  11m S  0.0  1.4   0:00.19 kerry
 3822 alx       16   0 29660  13m  11m S  0.3  1.3   0:01.76 kwin
 3909 alx       16   0 30344  13m  10m S  0.0  1.3   0:00.18 kpowersave
 3895 alx       16   0 24164  11m 9.8m S  0.0  1.2   0:00.32 ksensors
 3913 alx       16   0 28488  11m 9888 S  0.0  1.1   0:00.15 klipper
 3824 alx       15   0 28768  11m 9588 S  0.0  1.1   0:00.56 knotify
 3821 alx       16   0 27052  10m 8428 S  0.0  1.0   0:00.10 ksmserver
 3812 alx       16   0 26280 8220 6944 S  0.0  0.8   0:00.04 klauncher
 3807 alx       16   0 24216 7024 5832 S  0.0  0.7   0:00.06 kdeinit
 3829 alx       16   0 24928 6628 5392 S  0.0  0.6   0:00.01 kio_file
 3232 lp        16   0  7204 3372 1304 S  0.0  0.3   0:00.44 cupsd
 2519 root      16   0  4408 3032 1468 S  0.0  0.3   0:01.89 hald
 3810 alx       16   0 23392 2664 1524 S  0.0  0.3   0:00.08 dcopserver
 3992 alx       16   0  5720 2560 2020 S  0.0  0.2   0:00.04 gconfd-2
 4072 alx       15   0  4392 1892 1400 S  0.0  0.2   0:00.04 bash
 3445 root      16   0  4260 1732 1392 S  0.0  0.2   0:00.00 powersaved
 3491 root      16   0  5412 1732 1408 S  0.0  0.2   0:00.01 master
 3493 postfix   16   0  5412 1672 1364 S  0.0  0.2   0:00.00 qmgr
 3492 postfix   16   0  5380 1648 1344 S  0.0  0.2   0:00.00 pickup
 4100 alx       16   0  3756 1448 1004 S  0.7  0.1   0:06.03 ksysguardd
 3680 alx       16   0  3920 1400 1144 S  0.0  0.1   0:00.16 kde
 3969 alx       18   0  3656 1364 1132 S  0.0  0.1   0:00.01 firefox
 3000 root      17   0  3648 1252  980 S  0.0  0.1   0:00.00 kdm
 3399 root      25   0  4956 1200  864 S  0.0  0.1   0:00.05 sshd
 3325 root      16   0  103m 1044  772 S  0.0  0.1   0:00.02 nscd
 4275 alx       16   0  2188 1008  764 R  0.0  0.1   0:00.07 top
 2341 messageb  17   0  3420  996  732 S  0.0  0.1   0:00.16 dbus-daemon
da notare come a distanza di pochi minuti il quantitativo di ram usata dal sistema è sempre in costante aumento...che sia firefox il "problema"?

grazie ancora per l'interessamento