The HD 7770 performs similarly to the GTX 460 with almost half the power draw, that would be a good choice. I would grab the Pentium G620 for the CPU, it's cheaper than the Athlon and performs better (in games) with lower power consumption.
I wouldn't run the HD 5850 on a 300W PSU, the HD 6850 would be cutting it a bit close too.
300W is fine for running something like the Pentium G620 and HD 7770. They run just as well as the FX-4100 and GTX 460 or similar, that's fine for ultra high settings in most games. For the latest and greatest games (BF3/Skyrim/Crysis etc.) you might be looking at medium or high rather than ultra high but that's good for 300W.
Haha yes thats right, I understand with 300W is a bit hyperbole to mention "punch" but nevertheless i am expecting something like 460. Even lower a bit is okay. 7770 is out of my budget at the moment so i might lean toward 6850.
I just calculated above spec with PSU calculator and with 6850 min PSU recommended 238w and max load is 288w. Assuming 80Plus PSU, seems should be all right.
If i am going down G620 path for CPU, what is cheap reliable mobo to match with (approx. equivalent to planned mobo)?
I wouldn't worry about idle temperatures. So long as it's under 90c on load you're fine. The best way to prevent dust is to put filters on any intake fans, that should do the trick. Having more intake than exhaust helps too.