Overclocking (general) question

When I'm overclocking, do I need to keep the memory / core speeds at a 1:1 ratio? My stock speeds are 400 MHz / 400 MHz and when I've overclocked it with Riva to 460/460, but I often see people post their OC speeds, where the memory clock is higher than the core. Is there a different ratio that I should be increasing it by when I overclock? Thanks.
-Ryan

Nope, no ratio of any type required... just test each (core/ mem) individually to find their respective maximum.

memory clocks are usually higher than the cores because video cards using ddr2 and gddr3. What type of video card do you have?

It's a 7600GS (256mb DDR2). Stock = 400MHz / 400MHz --- I'll try increasing them independently. The max for the core seems to only be around 460. Any higher than that and I tend to get "snow" and screen black-outs. The memory may go a bit higher though.