According to Wikipedia Hypermemory is an ATI technology, much akin to Nvidias Turbocache. And from what I can tell they were both limited to use on the absolute bargain cards so that they saved money on memory by using system RAM.
I would expect your 9800GT to have either 512MB or 1024MB of onboard RAM and would not expect it to be using TurboCache (or even HyperMemory) at all, it truly is for the bargain basement cards only.
It may be that one of your memory sticks is not fitted properly (I am assuming you have 4 x 4GB sticks) or for some reason is not working properly. Are all four sticks identical?
What makes you think that this memory is being allocated to the graphics card?
Could you also try using CPU-Z and looking at the Memory and SPD tabs
On the SPD tab you should be able to see whether all four sticks are detected, the Memory tab should tell you if it is working in full dual channel mode rather than single channel or hybrid as might be the case if one of the sticks is not working.