I stumbled across your post and questions of a couple of weeks ago, and am glad you are interested in Linux. I thought this additional information might be helpful.
I use SuSE for my personal system and also for sysadmin on my SOHO. It is a very strong system, has very good documentation, there is a huge amount of rpm's, and a very strong technical community, At the same time, SuSE states that it targets "ambitious" and "power" users, and they mean that. One needs to be patient and do the homework, because there is a lot there and therefore a lot to learn. SuSE's default interface is KDE; gnome can be made to work but you give up a lot in SuSE without KDE.
KDE is the most widely used interface with Linux, 2x the next one, Gnome. KDE originated in Europe, Gnome in the U.S. KDE has more functions and applications than Gnome, It also is more granular, so you can control it at a very detailed level. There is a great deal of artwork for KDE, and many feel it has more of a Windows look.
Ubuntu I deliver to my clients. It attempts to "just work" out of the box, and is targeted primarily at new users. It is still a bit new, so the next release (about Oct) should be important. Ubuntu uses Gnome, which is more tightly integrated and has a more consistent look-n-feel that KDE. Gnome is somewhat mac-like and for a brand new Linux user, might be a bit easier to learn. Ubuntu is based up the very popular Debian distribution, which has more packages than any other by far. One small downside with Ubuntu is that they are very strict about not including any commercial or patented software. Thereafter, you need to perform a number of manual steps after installation to, for example, get the codecs to play MP3's. Ubuntu has outstanding user-level documentation and a fantastic community.
The suggestion about trying live-cd's is a good one. However, keep in mind that what you see is the default interfaces. With Gnome and in particular with KDE (which most use) you can literally make the interface look and behave anyway you want to . . .
you asked about the differences with Linux, and this is one of them. You have 100x the amount of control and flexibility with Linux. You can see exactly what is happening and why. You can change almost anything, if you really want to. A second (there are quite a few, I'm just sharing two) difference is free software - thousands of applications.
Finally, about the hardware requirements. The Linux kernel can run in very little RAM. There are a few distros like [bleep] Small Linux that have a custom interface with the kernel, and can be run from a memory stick. If you install SuSE and Ubuntu however, the biggest decision you have to make affecting hardware is the GUI. Both KDE and Gnome use approx 100MB. Now, there are other GUI's available with these distros, such as WindowMaker, Fluxbox, and XFCE4. These are lean and mean, and there is some nice artwork for WM, but they are not the large integrated GUI's that you may expect if you are comparing it to XP.
Hope this is useful. Give Linux a try. It's well worth the effort.