Pairing up Corsair with other brand RAM like OCZ

Hello all,

It may be a noob question but yeh i dont denie it, im a noob.

i now got Corsair 2GB (2x1GB) DDR2 1066MHz/PC2-8500 XMS2 Dominator Memory CL5(5-5-5-15)
in a P5Q Pro m/b with a E8400 al running at stockspeeds. i gess  

but i want to pair up another 2GB with my current rig, i want it as cheap as possible,
and as Corsair is quite expensive over other brands, i wanted to ask if its possible with my mb and cpu to get another brand and pair it up with the corsair, and if it will work?
I saw these were quite cheap:
http://www.ebuyer.com/product/119223 (Kingston 2GB (2x1GB) DDR2 1066MHz/PC2-8500 Hyperx Memory Non-ECC Cl5(5-5-5-15) Unbuffered)
http://www.ebuyer.com/product/142405 (Crucial 2GB Kit (2x1GB) DDR2 1066MHz/PC2-8500 Ballistix Memory Non-ECC Unbuffered CL5 Lifetime Warranty)

there are many others there on ebuyer, which one would you recommend me?

And what if i buy (2x1GB) DDR2 800MHz/PC2-6400 of any brand?
Would it work paired up with the Corsair 2GB (2x1GB) DDR2 1066MHz?
What would happen with the Corsair 2 GB, would it downclock to 800Mhz?
And can I overclock the new 800Mhz sticks with the Corsair 2GB to 1066Mhz?

All weird questions come up in my mind if I buy any other type/brand of RAM than my current Corsair 2GB (2x1GB) DDR2 1066MHz/PC2-8500 XMS2 Dominator Memory CL5(5-5-5-15)

Btw, I was looking for as cheap as possible, as long compatible and stable with my current RAM 

Hope you can help =))