Originally Posted by OS-Wiz
Some commercial desktops, perhaps Gateway, use proprietary parts, possibly the PSU. So first check to see if you can upgrade to a standard ATX PSU that will fit.
This is true, and you'll also have to make sure that you don't purchase a graphics card that will not physically fit in your OEM case.
These are just a few of the reasons why building your own computer is always preferable to an OEM build. You get maximum flexibility, and updatability, with a custom build.
HAF 932 Advanced / Corsair AX1200 / Asus Rampage IV Extreme / i7-3930K / H100 Cooling 4x120mm
16GB Corsair DDR3 / GTX 580x2 SLI + GTX 460 PhysX / SSDs Mushkin 240GB, Crucial 256GB, Crucial 512GB