Gigabyte memory PROBLEMS!

Test each one separately to make sure they both work.

I did, tested each one alone, then both together with inverse order, all the readings FROM THE BIOS give me 2gb of ram at first.

What make and model of RAM are they?
Also make sure (consult the manual for your motherboard) which slots to put the RAM sticks in. I'm assuming you have 4 slots for RAM. The boards can be fussy on the slots to use.

They're both 2gb 400Mhz DDR2 elixir rams, and I have 2 slots of ram lol, so this wont be a problem for me i guess

what windows you have ? beacouse xp 32 bit only support less then 3 GB, how much ram is in Bios ? or go to start, run and type dxdiag to see all info

Well, I took a look at the BIOS revs for your MOBO, (here 
http://www.gigabyte.com.tw/Support/Motherboard/BIOS_Mod...
and none of them lists any specific problems with RAM (although GB has never been known for being particularly astute about maintaining accuracy in changelogs...); have you done the obligatory "Load Optimized Defaults" from the BIOS since you installed it? This is a required step - one of the things it does is reads the SPDs off the RAM, and sets all the myriad 'auto' parameters in the CMOS to accomodate it...

manually set the voltage for ram, or use EPP settings if availage both for clock and power/voltage of ram

Having trouble finding Elixir 2G modules - can you post a part number? Found this snazzy little thing:
http://www.elixir-memory.com/download/Mini_Brochure_Eng...
but, unfortunately, it's a bit light on actually useable information!