If you plan on buying a new card in a year or less or you game at higher resolution, buying a 512mb version is just fine. If you want to own your card for 1 to 2 years going 1GB is almost a must need, future games will take advantage of it.

But honestly, most people will replace their card yearly, and you aren't going to see a huge difference that makes the 512mb card obsolete. You would be better off getting one with a non-reference cooler so you can clock it higher. And if you really game at higher resolutions, spend the extra money and get a 4870 X2.