Tiger has built-in spellcheck and dictionary/theasaurus support so all Cocoa apps in Tiger will have these features, including TextEdit. But Macs have been coming with Tiger for a long time now so if you are going to be getting a new Mac, it will have it.wait... so it's only on tiger now???
Personally, I find TextEdit suitable for many projects. It has many more features than NotePad or WordPad. It does not do everything, but if you need more you can get iWork for $80, or use one of the many free options listed in the thread. Personally, I like iWork because it has a lot of page layout features that I used all the time in AppleWorks. Most word processors don't let you do half the things Pages does in terms of page layout.