Can FSX Run at Peak Performance without an Overclocked System?

For years and with all versions of FS I have tried to run FS without any performance issues. I spent most of the time troubleshooting how to get better performance instead of toning and tweaking everything down. Someone in a forum suggested I build my own system, since my proprietary Gateway with an 17-920 ghz processor could not be overclocked.I took his advice and bought all the parts - ASUS P6T mb, another i7-920 2.67 ghz cpu, GeForce GTX275 1792 mb GPU, and 12 gig of Corsair memory, and Windows 7. We have tried everything, yet I can't seem to get a stable overclock. The machine restarts fine within Windows or when asleep but occasionally I get an overclock failure from a cold shutdown. Then a tech in a computer shop said to me that Overclocking is very tricky and I should be able to get FSX to run well without overclocking.Is this possible?? I have been under the impression that only overclocked systems run FSX well since it is mostly "processor speed dependent". Is it possible to change other components without the processor and still get top performance out of FSX? Thanks. Tom