the n900 multitasks fine before the overclock..overclocking makes it more responsive.

I would suggest to spend your time to be familiar with your new n900(with the stock firmware) for the first couple of weeks; so that you have some understanding how thing are(this will give you a baseline to compare) and also spend time in-between to read upon the overclocking thread, the wiki, the benchmark, plus time to setup your o/s for reflashing the n900 with stock image. This will reduce the stress level in case you have to flash your unit to recover from error.

cheers