Personally speaking, I started with a 5.1 PC setup myself (Logitech z-5300), and while I thought that was better than the usual PC speaker systems, it was still TOTALLY blown away by even a simple stereo setup. I couldn't stand mixing the center and rears of the PC speakers with my then stereo setup the difference was that noticeable.
You can start with a good pair of bookshelf speakers and a T-amp or gainclone. As upgrades, consider the following, in order of preference:
- Pair of floorstanders or another pair of bookshelves to serve as fronts (let your first pair act as rear / satellites)
- Subwoofer
- Center
Add in another pair for fronts if you want to experience side / rear effects, a sub when you want to get more of the bass, a center if you're having problems with movie dialogues and want to have voices solidly anchored in the front. Your choice.