give the difference

hi,
I want to upgrade my pc to pentium 4 so, there are some embigudies which i have to ask you to make my mind;

1) what is the difference between the socket 423 and 478?

2) In RIMM modules of p4 motherboards four slots are given 1,2,3 and 4, now tell me if i have one chip of 128Mb and other 64Mb how can i install them in RIMM module to have 192Mb ram,
and also tell me that can i use 2 chips of 256Mb in 1,2 slots and 2 chips of 128Mb in 3,4.

3)what is ATx factor, 6 layer pcb (30.5 x 24.4 CM)

the followed quries causes dillema to me and hope you will reply soon.

Yours Sincerly,
M.Faisal Ilyas.