The psu will be enough for your build, but I'd probably recommend at least the 500w version of the psu you selected because if you ever wanted to upgrade your video card down the line you'd have to also upgrade your psu to get the extra pci connector. You could also just use an adapter, but's it'll be more convenient with the extra connector.
Well you'll be pushing it hard...is it an old psu?
I would if it's more than a couple years old bump up to a 550 or 600 watt corsair.
There is nothing worse than having your system go down due to a power supply failure..
It's a relatively new PSU, and is the best budget unit at that price.
Ignore the listing that it requires a "500w or greater" PSU. The manufacturer lists that so even a poorly designed PSU can power the card. Total wattage of the PSU doesn't matter anywhere near as much as the power on the +12v rail. That is the most important consideration when buying a PSU for a gaming system.
The CX430 has a strong +12v rail @ 32a. It is far more than enough to power any video card that uses a single 6 pin (up to an HD 7850 or gtx 660) . It's also enough to power lower wattage 2-6 pin connector cards (7870 or 660ti) if you buy a y-adapter.
For a system using a single SAPPHIRE Radeon HD 6850 1GB GDDR5 PCIE (100315L) graphics card a minimum of a 500 Watt or greater system power supply is recommended. The power supply should also have a maximum combined +12 Volt continuous current rating of 26 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
You may find power supplies on the market that supply more than enough Wattage to run the system. However, some of them lack Sufficient Amperage capacity on the critical +12 Volt rail, which is necessary to properly power the critical components in the system (i.e. CPU and GPUs). This is the reason why graphics card manufacturers may overstate the power supply wattage, usually by at least 50 Watts, in an attempt to take into account some of those power supplies that have the weaker +12 Volt rail(s).
The Corsair Builder Series CX430 V2 (SKU# CMPSU-430CXV2), with its maximum combined +12 Volt continuous current rating of 28 Amps and with one (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with a single SAPPHIRE Radeon HD 6850 1GB GDDR5 PCIE (100315L) graphics card.