Re: Cannot boot into installation screen.
Are you trying to go directly to install, or booting a live system first? Sometime in the distant past I had better luck doing "Try Ubuntu" first, and then installing from there. You did not say what actually happened (blank screen? Did you wait long enough?). It can take awhile to boot from CD or live/install iso on USB.
I did use nomodeset when booting 64-bit 12.04 live/install iso from USB stick, but found that I did not have to use nomodeset on the installed system (which for now is installed on a USB hard drive to try it out). The live/install used nouveau drivers, and installed system automatically had nvidia drivers active. This was for desktop in my sig with NVIDIA GTX 550 Ti.
Strangely I did NOT have to use nomodset for 10.10, I HAD to use nomodset for 11.10, and I do NOT have to use nomodset for 12.04.
64-bit 12.04 iso on USB does boot without any parameters on my Acer tablet PC, but that is a 1 GHz 2 core AMD with ATI graphics (Radeon HD 6250).
i5 650 3.2 GHz, 8 GB, nvidia GTX 550 Ti @ 1080p (year old GT 430 began freezing) and assorted other computers