A:AnswerHello,
You can upgrade the GPU to basically anything that fits in the case. Just keep in mind that you would have to get a new power supply as well if you upgrade to more powerful GPUs.
A:AnswerHello, Yes you can add in another hard drive. You will need the necessary cables to install the new hard drive, and you would have to format it in your system.
A:AnswerHello,
This system meets the minimum requirements so it should be able to run the Oculus Rift S. If you are experiencing issues you may want to contact Oculus about why it is not working when the requirements are met.