Linux / Apache / PHP Scalability Issues

Hi,

We are having speed and scalability issues with our website hosted in LAMP environment. As i am new to Linux / Apache i am copying server related data here. Please let me know what i can do to imrove the scalability and speed of PHP scripts:

Current Memory Usage
                total           used           free         shared    buffers     cached
Mem:       3369064    3155716     213348          0     239616    2358752
-/+ buffers/cache:     557348    2811716
Swap:      4192956         68    4192888
Total:     7562020    3155784    4406236


Current Disk Usage
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             222G   15G  196G   7% /
/dev/sda1             251M   21M  217M   9% /boot
tmpfs                 1.7G     0  1.7G   0% /dev/shm
/usr/tmpDSK           485M  460M     0 100% /tmp

Following is statstics from XCache
 
Caches: - Slots Size Avail % Clear Compiling Hits Misses Clogs OOMs Protected Cached Deleted GC
php#0 8.00 K 64.00 M 36.81 M        no 83,690 985 10 0 no 976 0  
php#1 8.00 K 64.00 M 38.20 M        no 111,389 1,005 10 0 no 999 0  
var#0 8.00 K 8.00 M 7.97 M        - 0 0 0 0 no 0 0 29