|What's the purpose of this upgrade? Adding more RAM probably won't gain you any speed & adding a bigger HDD will give more storage capacity but it's doubtful you'll gain any significant peformance from that either.|
Kobian/Mercury lists at least 10 boards based on the Intel 845GL chipset & they all appear to max out at 2GB.
If you're still using the crappy Intel Extreme integrated graphics, you'd be better off adding a discrete video card...that will return the "shared RAM"back to the system. And although you apparently don't wanna do it, you should consider upgrading the CPU. The Celeron is a very weak & only runs at 400MHz FSB. From what I can tell, most the Mercury 845GL boards support 533MHz FSB, so look for a cheap S478 P4 533MHz CPU, but check the support list before buying anything.
For example, here's a 2.66GHz/533MHz FSB P4 for $15: