Memory upgrade D600

I've recently been given a Latitude D600, running at 1.3GHZ with 512MB PC2100 DDR RAM and a 20GB hard drive running at 4200rpm (which I intend to upgrade)

When I received the system, I repartitioned and reformatted (it had been set up as FAT; I changed to NTFS), reinstalled Windows XP Home and installed all updates, including SP3.

At first, with a few applications installed, it was running along at a respectable speed - much snappier than I thought it would - then, after I installed a few more apps, it began to get sluggish. Now it takes much longer to boot, load apps (especially Firefox), etc.

I'm not doing any gaming or graphic design, other than occasionally running MS Publisher. I've been careful not to load a lot of memory-resident stuff, and I've gone into the System Config Utility to make sure very little is running. I also regularly run utilities to defrag, clean the registry, remove junk and temp files, etc.

I'm running Avast as my anti-malware. I'm using Win XP's firewall; I was going to run Comodo, but it's sluggish enough that I didn't want to overload it.

I should mention - I did try an experiment recommended on Cnet - I had an extra 2GB flash drive, and configured Windows to use it as virtual ram. I didn't notice any improvement in performance, but it seems to me that's when it started to become sluggish. I've restored the default configuration (I think).

I was going to upgrade the RAM, but I've found posts on various forums stating that 512MB is more than adequate for my needs, that I won't see much if any improvement by going to 1GB. Will I see any substantial improvement in performance by putting in that much RAM, under these conditions and on a machine of this vintage, or is there no point?

Also, although most manufacturers (including Dell) are now suggesting PC2700 RAM for this system, PC2100 is still available, and I'm under the impression that PC2700 won't run any faster than the PC2100 speed, which is 266MHz. Is this correct?

It also seems to come in high and low density versions. Is this something I need to be concerned about?

Thanks very much.