Linux is designed to use all your memory first as others have stated. I went out and bought 2 1gb dimm modules. System runs way better and memory is relatively cheap these days.
I noticed you have sendmail, ssh, http, samba and kde running...
Personally, I'd kill all of that and run with fluxbox for ET... Atleast kill your network servers while your playing online games.
Know that doesn't help you too much but it's something to try anyway. I always got somewhat jerky performance with ET on Linux. Haven't tried it yet since I got a 6200 and more ram.
I have a 'Games' option on my grub bootloader and we all know what that is... Hmm... Kinda in the mood for ET now... Need a medic!!!