Will a brand difference in RAM sticks cause problem in my system?

If I understand this correctly, you now have 1-2gig Kingston memory stick?
You are going to install 2 x2gig more sticks? You do understand that DDR and DDR2 memory should be installed in pairs, right? It will not run in dual channel mode if it is not installed in identical pairs. So, if this is the configuration you have, pull the Kingston stick and give it to someone else who only has a single 2 gig stick. Install the 4 gig of Adata, and be happy. You'll never need more than 4 gig anyway.

If you have 2 x1gig sticks of Kingston, then you are good. And yes, the Adata should work in dual channel mode along side the 2 sticks you already have running in dual channel mode.
The system may not boot when you install the Adata due to too low of voltage.
If it doesn't, simply go into the BIOS and raise the voltage until you get to the point it will boot with the Adata, then raise it another .01~.02 volts to ensure it is stable.

Your PSU is no problem.

Mm... didn't knew about the logic of RAM, bloody noob I am.

I have a single 2gb's Kingston stick installed, so I'll take it out and keep the pair of ADATA's sticks.

Thanks so much for the advise jitpublisher

...So, will there never be a day when games ask for more than 4gb's of RAM?

.C.