Woah. Calm down dude. Obviously you need help because you are asking a question on this forum.
Ok, so the difference between the Athlon IIx3/x4 and the Phenom IIx3/x4 is that the Phenom II has an additional 6mb of L3 cache that the Athlon II does not. Think of it as superfast ram right on the CPU die. This will improve performance, especially in most games possibly up to 10-20% at best. For FSX , it being very CPU bound, I would recommend going with the Phenom II if you have he money.
Although I have not played FSX, I do believe a phenom II 810 with a radeon 5770 will perform just fine- albeit not on high settings. I have read about a core i7 920 and 5870 still not being able to max the game out with playable framerates on a 3 monitor setup. Which really bring us down to the questions:
-What resolution will you be playing at?
-How many monitors?
-What settings do you want? Do you want to be able to enable AA?
If you are able to answer these questions, someone who has played FSX or has more experience with it will be able to let you know exactly what you need.
As for saving for a Phenom II 965, might aswell. I would save up for the 0.8GHz difference. However the 955 at 3.2GHz would be more worth it, being unlocked you could simply up the multiplier by one in the bios and bam, it's a 965 at 3.4GHz. Unlike most games, FSX is very CPU bound. You would probably notice more performance going from an Athlon II x4 to a phenom II 955 then you would going from a 5770 to a 5870.
As for the power supply unit, it depends on the brand and model. Even though a bunch might all be labelled 460W, they would all have a different amount of power available on the +12V rails (the ones used for the Graphics and CPU). This amount is especially low on low end brands. For your setup, I would recommend a good quality power supply (corsair, OCZ, Antec earthwatts) of 500-600W. I would personally go with 600W because it would give me more room to upgrade.
I hope this helps.