Maximum PC

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Hi all, I have recently bought 3 video cards and returned all 3 in 7 days lol. First it was a galaxy gtx 570, then a evga 570, finally a HD 6950, the problem is they all power up but, I always get 1 long beep and 3 short ones, which means its a video display error. Now, when I first bought my gtx 570 last Sat. my PC booted fine I installed the new drivers, ran the PC hard Sat night and all day Sun for 12 hrs, gaming, benchmarks, blu ray movies etc. Now comes Mon, thats when I started getting the beep codes on boot up. So I replaced the card to a evga 570, then finally ati 6950 all from Micro Center, and still nothing, won't boot. My old gtx 285 is 100% fine, this is a weird problem, as I have more then enough power, here are my specs, any advice as to wether or not I might need a new PSU? I also reseted the BIOS cleared the CMOS take out a few RAM sticks to trouble shoot and still nothing. And whats even more weird is that the 285 is PCI-E 2.0 same as a 570. So now today, I'm going to get a 560 model this is my last shot then I will give up. So my PC is as follows:

2 LG blu ray burners WH10LS 30
1 seagate 7200 1 TB HDD
1 samsung 5400 2 TB HDD
Q9550
ASUS P5N-D
GTX 285
X-Fi Fatal1ty
7 120mm fans 1 80mm fan
PC Power & Cooling 750w <---to weak for a GTX 570 or a HD 6950 2GB ???
8GB Corsair DDR2 RAM
(Nothing is overclocked)

Thanks all.

Stupid qestion, but have you connected above 6+2 pin connectors to graphic card?

You have to research how many AMPS the video card needs to run...not watts. If the psu doesn't have the required minimum AMPS the cards won't run.

Nasty