Partitioning is helpful for the following:
1 - As Bebi indicated it is good to protect files so that if one partition has issues, you can repair the information on that partition without having to restore all your information (e.g., one drive with three partitions could have the operating system on the first 'bootable' partition, the second partition could have your installed programs and the third could have your personal files). Although, this is a bit complicated and not really needed for the average user.
2 - A second internal drive could be partitioned to use one or two partitions for scratch files (a.k.a. - virtual memory files) and a partition for backup purposes. This is how I use my system so I can set a program like Photoshop to use a dedicated partition on a second drive for scratch files. This does speed up the Photoshop program significantly.
As to your specific questions:"Is creating a HD partition helpful for a normal computer user?"
= Not really. Using an external drive for backup is a better use for normal computer users."Using a fairly new to brand new, high end pc, will it speed up performance a noticable amount?"
= Not unless you have a second drive and run a program like Photoshop."Is it helpful even for those who use their computer for e-mail and a few pictures?"
= Only for protecting saved files from accidental exposure to viruses or other issues that would affect the main partition that contains the operating system, programs, and personal files.
I would suggest saving some money and purchasing an external 'USB' drive to copy your saved e-mail, pictures, music, and other personal files to. This is probably the easiest way to protect your data.
Using a monthy (or weekly) drive defragmenting, disk checkup and virus/spyware scanning schedule is a solid way to keep your machine running smoothly and is an hour or so well spent. Just backup your files to an external drive and then perform the three maintenance operations.
I hope this helps.