Since I covered all the bases as well as posting here, it seems I've once again found the definitive answer: http://intel.com/design/chipsets/mature/index.htm
Down the chart in the memory section under i810 it definitely states that 512MB total is supported, so two 256MB should work. I also believe I know why HP kept the supported
maximum to 256M: http://answers.google.com/answers/threadview?id=333688
. They probably wanted to avoid any support calls that might arise from customers being "on the edge" since the system shipped with Win98 installed.
As for the PowerLeap upgrade, I went with the 1100/128/100/1.75 Coppermine Celeron since I'd already bought the PL-NeoS370 adapter, but I also have a 1400/256/100/1.5 Tualatin Celeron waiting for enough cash to get the proper PL-Neo370/T adapter for it. I may have a family member that would buy the current processor/adapter from me then.
As far a performance goes, it's leaps and bounds above what I did have. After I upgraded the BIOS and CPU, I had to run the i810 driver installation application and re-install DirectX 9.0c to get my graphics card finally working at full capacity. A co-worker gave me a monitor capable of 1280x1024 resolution and my 3D games look just like watching an interactive cartoon. (Just finished Soul Reaver 2, awsome!)