Although you can (sort of) get XP to run with 128 megs of RAM
realistically you should have 256megs as a minimum...

Be sure to check to see if your machine can take that much RAM...
if it's old enough that it had win95 on it...it may not be upgradeable...

If your machine is currently operating with 48Mb of RAM then the chance is
that it's quite old, and therefore it's likely that it will not run XP very
well, if at all, even with the extra memory. Unless you have confirmed that
this machine is fully compatible with XP, you might be better off to put the
cost of extra RAM towards a new machine.
--
Jeff Richards
MS MVP W95/W98