I'm building a new budget gaming rig around an HD6870 1GB, 128GB SS, and 8GB of DDR3. I haven't picked a CPU or mobo yet, but I'm leaning towards an i3-2120. I'm trying to keep my budget low, and I feel like something like an i5-2400 (or even 2300) might be more than I really need, or even get bottle necked by other components (correct me if I'm wrong, please).
I don't see why your 500w Silverstone shouldn't be great. I've seen i5s coming out Brand New and complete that had 500w power supplies. An i3 that uses 2GB of RAM and on-board graphics should run just fine on a 350W. Some brand new i7s with 4GB RAM and 1GB video cards come with 700w power supplies so consider that!
I'd send the new one back and wait till you need a stronger one and then think about it. Also you'd know by the time you have to upgrade to a stronger one exactly how strong you want it.
You will probably be just fine with the Silverstone. You might want to hold on to the OCZ in case something happens to the Silverstone during your build (never know), then send it back once you're sure you definately don't need it.
For a system using a single Radeon HD 6870 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 30 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
For a system using two Radeon HD 6870 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 45 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
The SilverStone Strider Series 500W (SST-ST50F), with its combined +12 Volt continuous current rating of 36 Amps and with two 6-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with a single Radeon HD 6870 only.
The OCZ ZS Series 650W (OCZ-ZS650W) will give you another 10 Amps on the +12V rail but it is still limited by its one 6-pin and one 8-pin PCI Express supplementary power connectors.