Have a look at the comments in this thread,

http://www.8052.com/forum/read.phtml?id=79811

where Kai Klaas sheds some light on the behavior of these internal crystal oscillators.

I personally prefer external oscillators, built more or less as I've described, which is very forgiving and value-tolerant, over the internal oscillators, but it's not because the internal oscillators aren't good and reliable, but, rather, because I often require the flexibility that external oscillators provide. I frequently find myself wishing for a different system clock frequency because of a number of reasons, serial comm's frequency not least of these. With the digital circuit I suggested, you can switch system clock frequencies back and forth under control of the firmware as needed.

One reason I favor jumpers as opposed to switches is reflected in Jan's comment, among others. A pair of 3-terminal jumpers will remove the second crystal from the circuit, thereby requiring only that you know the characteristics of the jumpers and the crystal, in order to calculate the required cap values.

I'm sure you know that you must be aware of the characteristics of ALL the components you introduce into the oscillator circuit of your MCU. That includes the cirtuit board material and traces, the sockets, the switches, and the passives, that you attach to the oscillator.

It is, of course, your choice how any or few of these factors you consider.

RE