[R] About Memory size

[Ferdouse Begum]

Hi,
I am trying to analyse cancer data set (affymetrix) by
using bioconductor packages. I have total 14 data set.
Total size of the data set is 432MB. Now I am trying
to analyse these data sets in my PC with RAM 512. But
if I want to get MAplot of my data set, I am getting
the messege
(
> MAplot(ptc.rawData)
Error: cannot allocate vector of size 73.8 Mb
In addition: Warning messages:
1: Reached total allocation of 503Mb: see
help(memory.size) 
2: Reached total allocation of 503Mb: see
help(memory.size) 
3: Reached total allocation of 503Mb: see
help(memory.size) 
4: Reached total allocation of 503Mb: see
help(memory.size))

Now how can I get rid of this problem? 
pls help.

With thanks

Ferdouse