A processor clock speed (eg. 2.6GHz) is determined by.
1. CPU Multiplier
2. Front Side Bus (FSB)
You multiply them together and you get the clockspeed.
So for example, that E-machines 2.6GHz celeron processor has a multiplier of 26 and a front side bus of 100. 26 x 100 = 2600MHz or 2.6GHz. Why is the FSB 100MHz? That's because the intel fsb is "quad pumped" i wont bore you with the details, but basically it means that you just multiply the actual FSB by 4 to get the final FSB.
FSB is the way the processor communicates with the rest of the system, thus, the higher FSB the faster your computer. But most importantly, the FSB is directly scaled to your ram speed. the faster your ram, the higher the bandwith it can provide.
There is something called a FSB/RAM divider.
At say a 1:1 divider, when your processor is a 400MHz FSB proc, the ram would run at 100MHz, if it were on a 1:2 divider, the ram would be running at 200MHz.
Take my processor for example. If i were to run my fsb at 200, that would be stock speeds for me as my multiplier is x12. 200 x 12 = 2400 or 2.4GHz. At stock speeds, i would run my ram at 1:1 since there's no higher ratio. Therefore, my ram would be running at 200MHz, if i were using PC3200, that would be stock. If i were to run my processor at 3GHz, or 250FSB x 12 = 3000MHz, and i were using PC3200, the PC3200 cant handle 250MHz, so i use a 5:4 ratio. Thus, bringing the ram back to 200MHz again.