I am considering the same thing, converting a laptop over to a Linux machine, but have not made the leap yet. If you can, post up how it goes for you, most here would be interested.
You can try most Linux distributions by just running them from a live CD.
In other words, you don't have to install them until you're sure which you like.
Just download the ISO file and burn it to a CD or DVD and start your computer from that. You can also boot from a USB stick if you prefer. That would allow you to remove one and install another using the same USB stick. Save money.
I have tried Ubuntu, Puppy Linux, Mint, and a few others that way. Those all have their own forums, and the members there are usually pretty helpful. Been a long time since I tried Mint, it will have changed a lot by now.
For a stand alone OS, right now it seems that Ubuntu has a pretty complete set of applications. Just my opinion, I have not made exhaustive comparisons. I really like that Ubuntu has LibreOffice.
For speed, Puppy is hard to beat. It runs in RAM. There are lots of Puppy variations, and it isn't easy to figure out which is best for your situation. But as Rridgely points out in that other thread, it is not as full featured. Also it is maybe a little bit less "window-ish" than Ubuntu.
There is a Piriform Forum topic about Puppy here, it has download links and a link to a pretty good Linux installer (called YUMI):
http://forum.piriform.com/index.php?showtopic=38610
Also there are some older Puppy Linux discussions on this forum, you can find them with a topic search.
Ubuntu can be downloaded from here:
http://www.ubuntu.com/download/desktop