[R] Increase Decimal Accuracy?

[Ann Hess]

Is there any way to increase the decimal accuracy for probability 
distributions.  For example (in R):

> 1-pchisq(90,5)
[1] 0

But in Maple, I find that the value is 0.67193x10-17.

I need to compare some really small p-values...is there a way to increase 
the decimal place accuracy beyond 1x10-16 (which seems to be the limit)?

Any help would be appreciated.

Ann