The 4870 is a quite powerful card although outdated and at that budget you cannot find a better card anywhere so i suggest you stick with that card and upgrade your CPU and mother board and RAM (if it is ddr2, if it is ddr3 your fine) for better performance.
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Shows your card is amoung the best and if a card is not 3 tiers higher then there is little performance increase that usually goes unnoticed.