This is just a guess, but the game is probably seeing that you have available memory so as it loads new things (areas, textures, possibly characters) it's caching them all for faster use later which is why the memory usage keeps increasing.

Using a lot of memory isn't a problem though, it's only an issue if your available memory gets low because then Windows will start using the pagefile (hard drive) as memory which is considerably slower. If the game is caching (and not leaking) it should be checking the available memory so it doesn't take more than it actually needs.

As a few others said in this thread, trying the non beta video drivers is probably your best bet (and selecting "Clean install" when you install them).