Unstable system - weak PSU?

Hello all,

I bought this PC:

CPU: Intel Core 2 Quad Q9550 2.83 Ghz LGA775
RAM: 2 x 2GB Transcend 1333MHz DDR3
Mainboard: GigaByte EP45C-DS3R
Graphics: GigaByte 9800GTX+ 1GB DDR3 PCI-e
Case: Eurocase ML550 - http://www.eurocase.cz/index.php?gid=706
PSU: Eurocase 550W - http://www.eurocase.cz/index.php?gid=650

I also use:
1 x SATA SSD
1 x SATA HDD
2 x ATA HDD
1 x DVD-RW

Here is my story:
I really like the case design, so I bought it with the PSU. Problem is that PSU has only ATX 24/20 pin + 4pin 12V connector, but mainboard requires 24pin + 8pin connector. So I used reduction from 4pin to 8pin.

After that the PC was functional. I installed OS (WinXP - I still hate Vista) with no problem. First problem occured when I tried to install TES4blivion. System freezes while instalation. Then I tried to install Far Cry, but it freezes while instalation too. After many attempts (and restarts) instalation of Far Cry succeded. So I tried to play it. Game went fine in menu, but when I started the real game, computer restart itself.

I tried many tests:
memtest86+ - OK
Hot CPU tester - OK
HDD tests - OK
3DMark2001 - OK
3DMark2003 - PC restart in the middle of tests

I expect no BSODs.

I suspect the PSU or the 4-8pin reduction. Everytime when PC is not idle (OS boot, CPU test, 3DMark runs) the PSU is whistling. I think that PSU is weak or crappy.

Do you have any idea what to try next to find out the cause of problem?

I am willing to buy a new and quality PSU up to 170$. Which type do you suggest?

thanks

Thanks, daship. I will check RAM more closely and let you know.