The Radeon 6870 is a far, far faster card than the GT 430, but both are previous generation cards. Something such as a Radeon 7750 (between GT 430 and Radeon 6870 in performance, way more energy efficient than both of them) for the low end or a Radeon 7850 for the upper end (well above the Radeon 6870 in performance, also way more energy efficient).
However, there are other factors to consider as well. For example, what PSU do you have? If the PSU is inadequate, then an upgrade to a higher power-consumption graphics card is not feasible.