A 64-bit OS should not have an 8 GB limit. A motherboard may list 8 GB as the maximum supported memory configuration, but that's not the same thing.
In the past, the addressable memory on a GPU had to be mapped inside the usable address space in your OS, but using a 64-bit OS has long since eliminated that as a common issue.
The motherboard's memory addressing limitations and the OS's memory addressing limitations are two different things. The memory on your GPU is not going to count against the total installable memory your motherboard can address, but it will count against what your 64-bit OS can address, however a 64-bit OS can handle significantly more than you have to be concerned with. Ultimately, you also have to consider the CPU, the device actually doing the physical addressing, and I think you will find that most modern processors can handle somewhere on the order of 8 terabytes of address space, so you should be fine for the near future. 🙂