Speaker to be used with PC

I have a desktop PC which PC has what I believe are called 1/8" jacks in back for speaker connectivity. Since I'm starting to build an Itunes Library, I'm considering racheting up to a better speaker system. Worth it or waste? Suggestions, if worth it. Thanx.
NHT M-00 Now at close out pricing: http://www.listenup.com/NHT+M-00-p-M-00-p-nht_cpu.html

KRK's entry level Rokit line would also be a good choice. As would JBL's LSR6325 or the soon to be released LSR2300 line.

I use the KRK VXT6 on my desktop; I'm quite impressed with them.
Buy an external DAC such as an MHDT Labs or PS Audio, or Benchmark and wait to find some Paradigm Active 20's (Active 40's if you can ever find them) and you have a great system. Someone just posted a very similar system recently, I am sure it rocks.
I use a Tivoli model 2.
I second Bob's suggestions, however, you are talking high quality with NHT and sound that is on par with most people's main systems.

The Paradigm Active 20's are also a great choice.

Both are main system killers or at least competitive with most mains.
Powered speakers by Audioengines (model 2.0) work great with iTunes.

I also love the Blue Sky EXO connected to an iPod. You can get this at Guitar Center for $200.
Probably heresy on this forum, but I still use the powered Bose Roomates I got in the 80's to plug into my sony walkman for darkroom use. They connect via headphone jack of my G5, seen them go for as little as $50 on that "auction" site.
I am using Cambridge Megaworks 510D and they working fine since 2001, also get a RealTek High Definition audio integrated or external soundcard