umair, your 800MHz FSB isnt really 800MHZ, it's quad 200MHz.
And the memory you have to use with it is the DDR (DOUBLE data rate) 400MHz. Which means the memory is clocked at 200MHz, exactly like your FSB.
If you use DDR333 (which is clocked at 166MHz) with a FSB of 200MHz, your setup suffers from asynchronous memory operation, in some cases severely.
Clear enough? I think you only *thought* you knew enough to make the right choices... asking ppl that actually make a living out of those things sometimes help. Be carefull with computer shop salesmen though like I said before. Because they sell the things doesnt mean they know how they work.