spatial correlation in lme and huge correlation matrix (memory limit)

Hi,

  I'm trying to introduce a (spatial) exponential correlation  
structure (with range=200 and nugget.effet of 0.3) in a lme model of  
this form: lme(ARBUS~YEAR, random=~1|IDSOUS).

The structure of the data is "IDSOUS" "XMIN" "YMAX"   "YEAR"   "ARBUS"  
with 2 years of data and 5600 points for each year.

I do:

corstr<-corExp(value=200, form=~XMIN+YMAX|YEAR, nugget=0.3)

with |year to calculate correlation of IDSOUS(plot) only within the  
same year since their positions did not change between the 2 years.

Then I try to initialize the corExp vector:

corstr<-Initialize(corstr, data=cover)

Erreur : impossible d'allouer un vecteur de taille 293.2 Mo

The RAM memory of R is too small to deal with my 5600x5599 elements.
So I've tried to use the bigmemory package to deal with these data,  
but nlme package is a SP3 class and a big.matrix object is a SP4  
class, so lme return to incompatible class.

Then I've tried the ff function, but it returns to this message:
Erreur dans UseMethod("Initialize") :
   pas de m?thode pour 'Initialize' applicable pour un objet de classe  
"c('ff_vector', 'ff')"

At this point, I'm out of idea. Is somebody knows how to deal with  
spatial correlation in a mixed-effect model and a huge correlation  
matrix at the same time?