If you don't know, then you assuredly need i386. i386 refers to your CPU's instruction set, and all 486, Pentium, Athlon, Duron, .etc .etc sets are I386/I486. Some of those others are for things likethin clientss, old Macintoshes and Amigas, or old SUN server hardware; Linux supports pretty much everything imaginable.
When you download, just get the small business card installer if you have high speed Internet. Debian will fetch everything you need right over your Internet connection, while skipping everything you choose not to get. If that's not your style, just grab the first CD; the other discs just contain various Debian Stable version applications which you can easily download after your computer is fundamentally set up.
Al;so, if you want something easy or more up to date, pick Ubuntu. Debian's best for set-and-forget server applications, and Arch Linux is a better choice for people who want to customize and really learn about their PC. Ubuntu shines at desktop useage with minimum fuss or setup.. Also, unless you put up with Unstable, Debian's packages will all hail from April 2007. Yes, 18 months old!