NTFS is not the right format for Ubuntu. The best course of action is to delete the 55GB partition in Windows 7 and leave it unformatted. Ubuntu will then find the empty space and create 2 partitions it needs there. One of then will be in ext4 format and the other one in Linux/Swap format.
(Update as of 2019
Recent versions of Ubuntu creates a swap file by default instead of a separate swap partition as described above.
Boot from the Ubuntu DVD/USB and you will get the choice of Install or Try. Choose Try Ubuntu. This will run it from the DVD/USB without touching your hard drive and you can see if everything, such as, sound, video, wifi, etc. work or not. When you are satisfied, you can choose Install Ubuntu from the live session. See: How do I install Ubuntu?
During installation you should get the choice of Install side by side Windows This is the dual boot option that will find the unallocated 55GB and do the partitioning and formatting for you.
The AMD part of AMD64 refers to the company who came up with the standard. It works in both Intel and AMD CPUs.
See: Dual Boot Installation with Win7 - Install Ubuntu in New Partition
Hope this helps