My processor supports FSB 800 only but my ddr3 4gb ram is 1333 FSB.

The memory speed and front side buss are not sync’d with a common speed. Yes, sometimes it works out that way, but not always. The engineers that designed the motherboard made allowances for this.
Your computers speed is 2.6Ghz. This is the rate at which it computes, 2.6 billion floating operation points per second. The Buss speed is how fast the computers motherboard can distribute commands and data from the processor, in this case, 800 mhz.
The memory speed is how fast the memory can turn on and off zeros every second, in this case 1.333 ghz (its slightly more complicated, but that’s what wiki is for).

Clear as mud?