my advice would be to get a good power supply. It is the one part where, when it fails, it could well destroy everything... It is the absolute last place to try saving money.
That and the advice to get a slower graphics card because you can always get a second later I also think is extremely iffy. Dual GPU setups need to consider heat dissipation and power. I don't think a 650W coolermaster is the best thing to run two 7850s on (maybe if it was a gold+ rated 650W antec or seasonic unit... but no PSU really wants to be running at too close to it's stated capacity if you want it to last well). And an antec 300 is not really the ideal case for multiple graphics cards either. It is decently ventilated, but dual GPUs is very difficult to cool, and will increase ambient temps by a lot. Your computer will need multiple case fans, and your CPU/GPU coolers will be running full tilt much of the time, so it will sound like a jet on takeoff.
Even the smallest donations help keep sc2sea running! All donations go towards helping our site run including our monthly server hosting fees and sc2sea sponsored community tournaments we host. Find out more here.