YouGamers published an eight-page long investigation on how much video RAM memory you need:
DDR SDRAM has gone through even more technological updates (we're currently on version trois, with DDR3) but along the way, GDDR has popped its head up. Without going into the long and convoluted reasons, this stuff isn't quite the same as the equivalent named DDR (for example, GDDR2 is more like DDR than DDR2, and there's now GDDR4) but it doesn't really matter. All that counts is that the latest video memory technology allows the chips to run faster, cooler and transfer and store more data.
So is speed everything? For graphics cards and 3D games, the answer is no.
Read on over here. The reporter concludes you shouldn't buy a graphics card anymore with less than 256MB of video memory if you want to run your games at max detail.