First of all, by demanding 3d game, I meant demanding for your current card, definitely not Far Cry, I don't even know if you'd be able to run Far Cry with decent framerates.(on your current card)
That message you got, it means that the memory clocks work apart from the core,they work in different cycles not pertaining to the rest of the card, timing signals are derived from special characters in the data stream itself, you will not be able to overclock the memory clocks.
By the way, overclocking your video card will not make the graphics clearer or anything, the main idea is to get it to run at faster framerates.
But a good core overclocking can go a long way, you can test your overclocking results via a system benchmarking program.
Heres one that I use sometimes, other than 3D Mark 2004.