According to this:
http://h71016.www7.hp.com/sp_main.as...moryconfig.asp
That machine has a limit of 128MB per dimm slot
Play around with it, as the DIMMs may still work. Unplug the machine and let it sit overnight, unplugged from the wall. And in the future when changing dimms, be sure to have the switch on the PSU (power supply) turned off and let the system sit for one minute before changing any memory or internal cards. Some boards are sensitive to these measures, others are not. Always better to play it safe.
You may also want to burn a bootable CD with some memory checkers on it to test your RAM before you spend a lot of time trying to get an OS installed with memory errors.
The only method I've used to create such a CD is using Nero, selecting "Bootable" cd, and pointing the program to a Windows98 boot disk for the sys info.
Bout time I upgrade that puppy to a Linux boot...