My N900 already runs at 1Ghz. (been doing this for over a year now)
Just overclock yours if you want faster clock speed using same CPU.

If they made an upgraded N910, it should be more like:
- Same Maemo5 OS
- OMAP 4430 (Dual-core ARM Cortex A9 + SGX 540 or better)
- 1GB RAM
- 4" or bigger Resistive LCD (higher res welcome)
- Bigger battery
- Proper USB host support
- HDMI out (at least 720p output)
- HD Video playback / recording

Now that would be my dream mega-N900.
If Nokia made that, I wouldn't go to any other phone for a LONG time.