Upgrade to ram for a D420

I have a Latitude D 420 with 512 dedicated and 512 other ram memory.  It is my light weight travel computer.  It seems to run a little slow.  Am thinking of upgrading the non dedicated ram memory to 1gb which seems fairly easy to do.  Any body have thoughts about this.  Does any one have a 420 with 1 gb and how does it perform.  I run win xp and do not plan on upgrade to vista.