Thats how most people have them, Its fine.
Google "my graphics card broke my motherboard under the strain of its weight" or something and see if you find anything...
Or find some old broken hardware and see how much weight it takes to break a PCI slot or something

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2500K 4.4ghz,P8P67Pro,120gb Force3, 12gb Vengeance,HD6950,TX850v2,W7HP,LaCie Blue 22" crt COMMODORE 64 Silver Label 1Mhz/128D 4Mhz(Z80)