memory always used at 99%

Hi
I'm experiencing something I see as a problem.
I run two computers with freebsd : a server with 5.2.1 and my computer
with 5.3-beta-4
On the two version (and as far as I remember, with 5.1 too), the memory
is used at 99% after a few hours.
For example, here is the mem. use after only 3 hours :
Mem: 318M Active, 472M Inact, 120M Wired, 20M Cache, 111M Buf, 63M Free
I suppose it's a problem for two reasons : music is experiencing some
weird effects or distorsions (the same thing when I have a load of 2 or
3) and my computer used to crash when I was trying to launch UT2004 in
this situation. There was not that kind of problems when I was launching
it just after the boot. Another weird fact is that when I kill quite
everything to have the same configuration as at the boot, the mem use
change only a few and the mem free is at about 5% only ...
So my question is : is it normal ? If not, did I something wrong ? If I
did, what and how to correct it ? (or even a URL with a
solution/explanation)
I just hope It's not the wrong list :)
Thanks a lot !

-- 
Gregory Nou <gregorynou_at_altern.org>
Supélec