Just go shop for PC speakers. All PC speakers use the headphone connector. There is a HUGE range of options out there from cheap $10 to $100+ sets. You should be able to find a 2.1 (2 speakers plus subwoofer) for anywhere from $20 on up. I have these:
(Click the pic for more info)
They sound great, produce way more bass than I'm comfortable annoying my neighbors with (but the sub has its own volume so you don't have to annoy them), and they're really solidly built. They go for about $80.
Before these speakers I just had a set of $25 no-name 2.1 speakers from WalMart.
One of the cool things about the Z4 is that it accepts an auxiliary input (headphone input) that does NOT mute the main input. So you can connect your ipod to the speakers and still be able to hear your PC through them. I had my Xbox 360 set up on my desk using my PC monitor for a while and had the 360 running through the speakers' aux input. It allowed me to play music from my PC while still hearing the Xbox 360 sound. Really convenient.
-The MinisterofDOOM 
|The Q of DOOM| - |The Maxima| - | Hear my Q!|
4.08 VLSD, NICO 8-way ECU, full exhaust with custom headers,
Tokico Blues with Eibach springs, Stillen FSTB, 20mm RSB
