I don't really know how accurate that calculator is, but I would tend to accept its recommendation in this case. As 6850s are not power hungry at all, so people recommending 600-650W PSUs are overestimating power consumption, or possibly they believe that all units below that wattage level are not high quality.
So yes it will definitely work and there won't be any problem adding HDDs or SSDs (as long as you only use ~5 of them, as that is how many SATA connectors the EA-430D has). It would be fine with any graphics card that only requires one PCIe connector. Which I suppose could be quite restrictive as for instance 6870s aren't always that much more expensive than 6850s, but that requires two PCIe connectors, which might be beyond a 430W PSU.
I would like to share my experience at this point. The first GPU I bought was a 4890 which is relatively power hungry and requires two PCIe connectors. Soon I will be replacing it with a 6850 (mostly to reduce noise levels), it is less power hungry, cooler but it outperforms 4890s. So I think it will be entirely possible for you to remain in the 6850 power level and be happy with GPU upgrades.