I'd stick with 2.1 for a few reasons:
1. No need for a USB device = more money to spend on better speakers
2. 2.1 speakers are generally cheaper than 5.1 so you will be able to get better quality for a better price
3. You'll only need 5.1 for a home entertainment system that you'll want to watch a lot of movies on, if this is the case then the eMac doesn't really have the screen for that kind of thing anyway.
Trust me, go for quality over quantity, after all, isnt that why you're with Mac in the first place?
Have a look at Harman/Kardon Soundsticks II, I'm still totally impressed by these speakers and love them to bits, if they're a little too expensive, go for the JBL Creatures if you can, a little less power and not quite up to the standard of the soundsticks but for their price they are very good quality speakers. You'll do good to find speakers of the same or better quality for the same price.