The iMac 27" (2011) model with EMC 2429 has a proprietary MXM graphics card format, which means that finding compatible graphics cards can be challenging. Apple used custom firmware on their MXM cards, which restricts the compatibility of third-party options. However, there are a few options you can explore:
* AMD Radeon HD 6770M: This is the stock graphics card that came with the iMac 27" (2011) model. If you are looking for a replacement, you should be able to find used or refurbished cards that have Apple firmware.
* AMD Radeon HD 6970M: This is a slightly more powerful option than the HD 6770M and is also compatible with the iMac 27" (2011) model. You may find used or refurbished cards with Apple firmware.
* NVIDIA GeForce GTX 675MX: This is a third-party option that is known to work without requiring a firmware flash. However, it's important to note that this card is not officially supported by Apple, so there may be some limitations or compatibility issues.
Upgrading the graphics card in an iMac can be a ***complex task*** and may require technical expertise. Additionally, since the iMac 27" (2011) is an older model, the availability of compatible graphics cards may be limited.[br]
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I hope that's help