A 4870 is a 4870, in general. There can be an overclocked version of a card, where the manufacturer overclocked the 4870 and guarantees it at the increased speed, but, in general, a 4870 always performs the same.
The other key, though, is the memory on the card. You said you saw a 512MB version. That would not perform as well as a 1GB card. Most 4870's are 1Gb cards, not 512MB.