Just checking, but are you using the mic in on your computer or an actual 3.5 stereo in? The mic in is only mono and is very quiet. You have to change your audio input in you computer to mic and adjust the gain to hear it. That is not a good way to run audio, you'll get alot of hiss. If it's a real 3.5 stereo jack you're ok. If you don't have one, you can buy and iMic rca to usb converter that is around 30 bucks and works great. I use it to convert records or tapes to my computer when I don't want to use my m-box.
Niode is dead on about the options and the note about quality is what you pay for. Matching impedence is important in getting a quality signal that is of sufficient volume. The one thing I'd mention is if you go to a mixer you'll have to buy a rca to 1/4" adapter so that your Dreamcast, 360 and whatever else you use can be hooked into the mixer. Each system will have two signals, a left and a right which will take up one channel each on your mixer. So a small 4-channel mixer will only accomodate 2 systems.
Just something to keep in mind.
I'm thinking you don't need to put all of them into the computer at once so a switch box should be sufficient. Something like
THIS would probably get the job done. Or a higher end one like he suggested would work great.
If you plan on doing most of your gaming on a pc or routing your systems into a computer, I would probably bite the bullet and buy a good sound card that has a sp/dif (digital input) and a good reciver you can plug everything into, regulate to volume and send to the computer via sp/dif.
The way have my game systems hooked up is to a/v switch boxed with 2 audio outs. I send the rca's to my tv and the 3.5 to my reciever, which I can then run into my computer. It works pretty good and quality is great. There is a little bit of hiss, but it's barely audible and unless you doing professional applications it works just fine.