Well in many cases the Dark Knight 4870 will be faster. Still games like GRID already show a benefit from the extra RAM and future games are likely to continue hogging up more and more vRAM. I would spend the extra money to get the 1GB version, but that 512 is still a good choice so it's up to you.
I seen (from personal experience) a textured modded Oblivion easily fill up my 8800 GTX's 768 MB of VRAM at 1680x1050 with all eye candy on. $20 isn't much for a little insurance with newer more texture intensive games - and possible larger monitor purchases.
wow, ok ill get the 1 gb version, most the reviews ive seen said it wasnt actually any faster. i have a 19inch widescreen samsung lcd, its like 1444x900 or something res, so ive never been able to play anything at max settings before