I had similar trouble when installing Ubuntu on some P3's at the small private school where I teach and serve as the de facto IT department. At some point in the installation, I'd get a black screen with a cursor. After letting it sit for hours with no change, I'd thumb the reset button. It runs through the boot process, then gets to the part where it's going to draw the GUI. Then the screen would just go black and stay black.
I'm new to linux, so I called up a friend and we stepped through some stuff. I'll let you know what we did in case it helps you, too.
My problem turned out to be that the P3's have onboard graphics and a PCI Nvidia TNT2 (where the monitor is actually plugged in). Ubuntu recognized the onboard graphics and apparently quit looking. We had to download the Nvidia driver and then edit the xorg.conf file.
During bootup, I hit ESC to launch Grub. I chose to boot to recovery mode, which after an unholy length of time gives me a command prompt. "Login" to log in as the user I created during installation. apt-get the nvidia drivers, then cd to the /etc directory. sudo nano xorg.conf
I added another device. Description can be whatever you want, apparently (I chose "NVidia TNT2"). The driver name is just "nvidia." My friend said the PCI address (or somesuch) didn't matter, Ubuntu would figure it out on its own. Fair enough. Then down to the "Screen" section and change the primary display device to my "Nvidia TNT2." Write out and exit.
It's not a complete success, but it's progress. Ubuntu flashed an Nvidia splash screen, gave me the twirling-cucumber-slice looking mouse pointer, then quit sending talking to the monitor (monitor goes in "it's not my fault" mode -- displays a box saying that the monitor is fine but receiving no signal). SHFT + CTRL + F1 (or is it ALT + CTRL?) to switch over to the console, which reveals that Ubuntu is downloading an hour's worth of updates.
I had to leave work at that point. I'll let you know how it looks tomorrow.