Problem on startup

Hello, I got my new computer a few weeks ago, when I got it the technician told me there is a problem and that the GFX and the Motherboard don't feet properly (which actually was BS) and he said that because of that when I boot my computer I'll get a black screen, and then wait till I hear this beeping sound and then restart, so now my machine works like this, when I boot I see nothing (the motherboard doesn't recognize that there's a gfx connected) and then after the beeping sound I restart and it works, but it works badly, when I am playing games my computer doesn't perform as well as it should, I tried buying a stronger PSU (had a some unknown company 400W and replaced it with Coolmaster eXtreme 500W), I tried looking inside, everything looked alright , the gfx fitted inside the Motherboard , a wire that comes out of PSU into the gfx, I've tried downloading new motherboard and gfx drivers and it did not help, can any of you indicate what is my problem?, I am really pissed off that my computer performs like this , here are my specs.

GPU : Twintech GeForce 9600GT 512MB
PSU : Coolmaster eXtreme 500W
CPU : Core2Duo E6750 2.66GHz
MotherBoard : GA-P35 DS3L Rev. 2
Sound Card : Along with the motherboard.
OS : Windows XP SP3
Ram : 2x 1GB 667MHz.
MotherBoard's drivers up to date.
Graphic Card's drivers are up to date.
Soundcard drivers are up to date.

Thanks in advance.