MP 4,1: 8+8+2+2 a good ram config?

I read here and there that this system is best done in 3's for some odd reason.
So why have 4 slots?

I currently run 2-2-2-2 and with LR, PS, DW, FF, etc etc. the system is grinding to a halt everyday.

Wondering if I get a pair of 8's and keep the deuces, will the pair of 2's be a hindrance or a boost?

If I get 3 8's will the extra 2g card in the 4th slot add or take away?

Just trying to gain a better understanding of how the config. works and why it's all uppity LOL

thanks!!!