Bad performance on a low memory server

Hi, if I posted this in the wrong group please correct me :-)

I have a virtual strato server (debian 4) wich I use to host 4 sites
that get about 10.000 / 15.000 hits a day. I didn't really expect any
problems since a vmware test with the same setup could easilly handle
everything.

Lately im running out of memory (and so is ther server). I get all kinds
of errors like cant allocate momory (in the unix shell) and cant create
mysql socket and plain refusals of the site to load.

So im trying to tweak apache (and php) but im not really succeeding
since most of the howto's assume lotsa RAM (I could allways upgrade or
get a real server but that wont help me undersand why and how)

Can someone give me some pointers please on what would be effective to
adjust(and what not).

Ive used this howto to set up my server but i skipped everything but the
php mysql apache stuff:
http://www.howtoforge.com/perfect_setup_debian_etch

memory info from top:
Mem: 2070020k total, 2057596k used, 12424k free, 207696k buffers
Swap: 3068392k total, 76k used, 3068316k free, 1142032k cached

also see www.chantalkarel.com for phpinfo()

Ive tried disabling all he mods I can find and lower the php memory
limit etc but I have the idea im missing the essesintials .

If there is any info you need ill psot it offcourse :-)

--
Arjen
http://www.hondenpage.com