I just went from 4GB of memory and added the 4 GB 800 MHz 240-pin XMS Dual Channel DIMM. Everything is so much faster with the added memory. My boot time went from about 8 minutes to under 2 minutes and now I'm usually around 32% memory use, even when I'm running lots of programs. The browser is faster and streaming videos run way smoother.
Worked for weeks on troubleshooting a PC that kept hanging up. All the diagnostic software programs could not resolve. Purchased the 4GB PC2-6400 DIMM DDR2 Memory Module w/Classic Heat Spreader and resolved ALL software and hardware issues with it's installation!
Very good memory, but not compatible with Samsung memory that came with Dell XPS 420. Conflict reset memory speed in BIOS to 667 with both installed. Samsung in slots 1/2 and Corsair in slots 3/4. Now only use Corsair in 3/4 at 800. Tech support could not explain why Dell recommends this for XPS 420 when there is a conflict.