ROG Striker 2 formula overclocking help

[jnjg]

hey all, first time overclocking and i could really use some help
i have a few socklet 775 computers with q9650s and id like to get the best performance out of them.

they are all different mobos so i will just focus on the main one

Striker 2 Formula - updated to the latest bios in order to get the q9650 to work.
memeory OCZ Reaper HPC DDR2 1066 2gb x 4
cpu q9650
gpu HD 6790(im only using the 6790 because its the best i have atm.) - would anyone recommend 2 video cards to use sli? with this mobo or would it even be worth it? it can hold 3 video cards but small video cards ,so i figured 2 bigger/newer ones would be best.
psu 850 Seasonic m12 bronze

id like to overclock my cpu and get my ram at 1066mhz at the moment speccy displays 400mhz, but i think thats 800mhz becasue its doubled?

any help would be much appreciated thanks.

Update, i dualboot xp32bit and windows 7 64bit is overclocking a bad idea since xp only uses 4gb of my 8gb ram, really wont need to be overclocked for xp games but having it overclocked while using xp could that damage anything?

i know this is an old system, but i would like some newer games to run at decent fps when running on windows 764bit like the new Unreal tournament, gta 5,mortalkombat 10 ect, i dont expect to be able to run everything. but it would be nice to have the system at its highest level.

I feel like not many will take me serious for trying to OC an old system but if I understand correctly most new mobos probably dont support windows xp or am i mistaken? so i dont want to start on a new rig yet, xp can run alot of games the newer OS cant, plus its the last OS to come with IPX which MANY old games require* for multiplayer, the computers i have are the perfect middle ground for games from 1997ish to 2015.