I have had the same problem when adding 2GB to the existing 2GB in a Dell XPS machine. The solution is to modify the boot.ini system file by adding the /3GB switch.. This can be a rather hairy procedure and I suggest reading thea article at this addrss: http://support.microsoft.com/kb/291988 It is also helpful to follow some of the links in the article for explanations of the usage of memory by XP.I hope this helps.
The /3GB switch has provided a solution for me in the sense that the "Real memory available to Lightroom" is reported to have increased to 1228.8 MB and there does appear to be an improvement in performance.
Good. Although you have to monitor the 'real' memory usage as reported by LR to see if it makes a difference. I'm pretty sure though this has nothing to do with you adding another 2GB of memory to your Dell computer...