32 vs 64 bit

I ran  R on an AMD Athlon 64 with a 32 bit Linux (by accident, I moved
the hard disk from another machine). Now, after install 64 bit Linux and
64 bit R, I am finding some of my R tests are quite a bit slower, I
think because memory demands cause a lot of swap. Does anyone know if
extra memory usage would be mainly because of R itself, or is there a
lot of extra memory demand because of the OS, windowing system, and
other apps?

There's a definite effect from R itself.

I never run a machine without plenty of RAM for everyday tasks but if you 
did, that could have a bigger effect on a 64-bit OS as all the 
applications are larger.  (My 64-bit Opteron desktop has 2Gb, for example, 
whereas its 32-bit predecessor had 1Gb.)

This _is_ discussed in the R-admin manual (section 7 in R 2.2.0 in the 
version I just looked at).  You can run 32-bit R on 64-bit Linux if you 
want for speed (and I sometimes do): this too is discussed in the manual.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595