Need ram for dumpster find.

Solution
I just found the memory support from HP's website on that PC:

System Memory - Single Channel Configurations
128 MB DDR SDRAM PC2700 (333-MHz) non-ECC
128 MB DDR SDRAM PC3200 (400-MHz) non-ECC

System Memory - Dual Channel Configurations
256 MB (2 x 128) DDR SDRAM PC2700 (333-MHz) non-ECC
512 MB (2 x 256) DDR SDRAM PC2700 (333-MHz) non-ECC
1 GB (2 x 512) DDR SDRAM PC2700 (333-Mhz) non-ECC
256 MB (2 x 128) DDR SDRAM PC3200 (400-MHz) non-ECC
512 MB (2 x 256) DDR SDRAM PC3200 (400-MHz) non-ECC
1 GB (2 x 512) DDR SDRAM PC3200 (333-Mhz) non-ECC

^^So it can take 512MB modules after all.
I had two Samsung Rambus 256MB modules in my P4 2.26GHz Northwood socket 478 build (which I still have in pieces). What I can't remember is if that was a RAMBUS only memory limitation or if the motherboard DIMMS allowed DDR 512MB modules. For the period though, 512MB memory chip max capacity in a DIMM sounds generous.

Off topic - I'm still kicking myself for not selling those modules circa 2010 or so when they were selling on eBay for $100+/stick. If I remember correctly, engineering labs were wanting to upgrade their Dell workstation Rambus builds to 1GB or more in memory and snapping them up.
 
I just found the memory support from HP's website on that PC:

System Memory - Single Channel Configurations
128 MB DDR SDRAM PC2700 (333-MHz) non-ECC
128 MB DDR SDRAM PC3200 (400-MHz) non-ECC

System Memory - Dual Channel Configurations
256 MB (2 x 128) DDR SDRAM PC2700 (333-MHz) non-ECC
512 MB (2 x 256) DDR SDRAM PC2700 (333-MHz) non-ECC
1 GB (2 x 512) DDR SDRAM PC2700 (333-Mhz) non-ECC
256 MB (2 x 128) DDR SDRAM PC3200 (400-MHz) non-ECC
512 MB (2 x 256) DDR SDRAM PC3200 (400-MHz) non-ECC
1 GB (2 x 512) DDR SDRAM PC3200 (333-Mhz) non-ECC

^^So it can take 512MB modules after all.
 
Solution
likely answer is comparing all the 865GV chipset motherboards (and I looks only at intel models) and all were 2GB max but all where 2 slots motherboards.

Realistically , 4x512MB = 2Gb total is probably right.

I checked
Intel® Desktop Board D865GBF
Intel® Desktop Board D865GLC
Intel® Desktop Board D865GRH
Intel® Desktop Board D865GSA
Intel® Desktop Board D865GVHZ