http://www.ubuntu.com/getubuntu/upgrading is their official page. I didn't even know about the Long Term Support releases.
If you're planning on sticking with one version for a very long time and see no need to upgrade to every new version, and everything suits your needs, I'd stay with 8.04.
I personally am an early adopter who enjoys living dangerously; when a new release comes out, I will backup, reformat, and replace my .home directory after the new OS reinstall. Apart from SUSE 11.1, I've never had any issues with new distros being buggy or so bad I had to downgrade, and the new features and revisions tend to have at least one little addition I've appreciated.
But especially with 9.04 about to be released (April 23?), you might want to wait. Especially if 9.04 will be a LTS release as well (I currently do not know, but a brief google search suggests it will be a LTS) I'm curious as to what new features will be implemented (Ext4 might be one of them, which is claimed to be faster than Ext3, but that's just a vague memory right now.)
I can say that Ubuntu 8.10, when benchmarked against 8.04, is marginally slower (from milliseconds to several seconds depending on application). Performance being #2 in my book, you might want to install and stick with 8.04.