Should I get a 4870 512MB or 1GB?

Depends on what the price differance is as to if its worth it, it wont increase your performance FPS wise but it would help in games with lots of textures.
Im in the UK so from my point of view if the card was £150 for a 512 card or £170 for the 1GB card then i would probably get the 1gb card. The times it makes a differance wont be many but it will make it more usable into the future. If you dont plan to keep it for more than a year just get the 512 card but if its an investment you want to last a couple of years get the 1gb card but dont pay lots extra for it.

Mactronix

I remember reading about the 3870 1GB card and that it was not worth it because it used lower quality RAM. Does the 4870 1GB use the same RAM as the 512MB?

The biggest advantage that the 1GB card will give you is the ability for higher quality textures and more AA without taking a hit. Look at the 4870X2 compared to 2 4870s in CF. Since each 4870 in the 4870X2 has a 1GB frame buffer when AA is put on or higher it takes less of a hit where as CF 4870s will.

I say the 48701GB. I have a HD2900Pro 1GB and its served me very well.

No and yes depending on how you look at it . The ram that 1GB version uses is a high density ram. It's like dual layering the memory to achieve 1 GB. However, the type of ram is still GDDR5 (same as 512mb version). These rams are brand new at this point in technology and so they're nothing near medium or low quality.