I've got a Creative 7.1 system which was pretty cheap (around £100 IIRC) but, honestly, it works surprisingly well.
I've got a Bose Hifi (over £1,500) and the sound from the Creative speakers is probably as good as the Bose system.
No idea what Creative speakers are like now but these ones all have fairly substantial chipboard enclosures and good quality doped-paper cones.
The biggest issue I've seen with small speakers (such as those used in PC set-ups) is that the housings are made from thin plastic and they'll vibrate, causing noise, and they're also impossible to mount solidly... which creates even more vibration and noise.
My advice would be to try and look at some dedicated 5.1 and 7.1 systems in person to get an idea of how well made they are.
Once you see one that's decent quality, erm, go buy the same thing off Amazon or eBay for the best deal.
Thing is, a poorly made speaker system will be horrible.
A decent PC speaker system will be designed to work with a PC (duh!) and it'll be, say, 80% as good as a pukka hifi system.
You can go to the effort of buying an amp' and hifi speakers and then hooking it up to your PC but (assuming it all goes according to plan) it's only going to be 20% better than the decent PC speaker system.
Also, as a rule I tend to find that it's often software issues that detract from PC sound, rather than issues with the speakers.
Basically, if a game sounds good on a decent set of PC speakers it'll only sound a bit better with a proper hifi system.
Conversely, if a game sounds terrible, it'll probably be a software issue and spending more on better speakers won't help.