Just because I couldn't find anything solid on this anywhere online when I was searching...
TL;DR - the dx2450 (and, I imagine, similar business class compaqs) can
support more than 4GB RAM - at least 6GB, though possibly 8GB.
I have an old HP Compaq dx2450 Microtower that I picked up a few years back. It's a business-class machine that I threw a cheap gfx card into (Radeon HD 4670 passive) and used for playing WoW so as no to stress my macbook pro. (For web searchers, the model is d2450sM/A4450/160hnqm/1T/16rf UK - gotta love those catch HP model numbers ;)
Anyway, the point. This had been running away happily on 4GB RAM (4x 1GB DDR2) for some time. I had contemplated increasing this, but any specs I could find reckoned the machine maxed out at 4GB, even though they also stated it would recognize 2GB sticks. creatorofsplab
recently gave me a pair, allowing me to test.
The BIOS immediately recognized a total of 6GB (2x2GB + 2x1GB). Windows XP didn't (obviously), so I upgraded to 7 Enterprise, under the incorrect assumption that MS would not release a 32-bit version of their 'enterprise' class OS. *sigh* A few days later, after getting hold of an actual 64-bit install of 7, I confirmed that the full 6GB of RAM were recognized and useable. (I stressed the machine to using over 4GB of active memory by loading a ton of apps simultaneously just to be sure.)
The type of RAM used is a mix of crucial and corsair - a matched pair of 1GB and a matched pair of 2GB. I have no way currently of checking if it will support 8GB correctly, but will update here if I ever do.