I am running a homebuilt with OEM software. One of the reasons I didn't go with the HP is the difficulty in upgrading the memory. I see that there is some postings on how to do it, but it isn't easy. You really have to tear it apart.
With the 1024 of memory, I am now running several add-ins, a defrag program and even added Sharepoint Services 3.0 without problems or noticeable slowdown. Off all the upgrades I have done on computers, this one provided the most obvious benefit.