Welcome to TSF. As long as you have a decent heat sink and cooling setup I think your OK with what you had. I'm surprised you were able to overclock a Duron by 400mhz. If your system runs stable and your temps are OK your in good shape.
Have you viewed ASRock's website for CPU upgrades.
http://www.asrock.com/mb/overview.asp?Model=K7S41GX. The Hybrid Booster overclocking feature. CPU support upgrades are to the left of your screen under CPU support list. Something to consider. By the way your English is fine.