NVIDIA is the obvious choice for various compatibility considerations. Though you also have to look at your PC's PSU, connector-cable(s) and possible pin-adapter, motherboard slot size, possible new type of monitor cable needed etc. Also note that Windows 7 drivers do not support 40 series NVIDIA cards, and that so far as I know Poser 11 didn't support certain capabilities of 30 series cards or higher. 

The sweet spot in power/price is probably an NVIDIA 3060 with 12Gb of onboard RAM. That would also allow you to do local AI image-generation in the future. Around £250 in the UK.

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