I made a picture of a video card to show you what I'm talking about
:

the PCI slot is 32-bits wide, which means it transfers 32 bits of data at a time to and from the card.
the memory bus is a completely different thing. it transfers data to and from the GPU and memory (VRAM) and on your card is 64-bits wide.
the PCI bus being 32 bit doesn't affect the memory bus being 64-bit.