Do NOT bleach. You need some retr0brite. The yellowing is due to bromine iirc, it's built into plastics as a flame retardant.
http://retr0bright.wikispaces.com/
retrobrite is a dumb hipster concept made by bathroom chemists and made popular by retarded Amiga/C64 enthusiasts that glom onto anything that might somehow relate to their computers. Half the ingredients are useless. Xanth gum, glycerine? Who cares about that shit. That's to get it to some form of goopy consistency so you can make a mess and get two-toned results.
The key ingredient is the peroxide. Even the oxyclean is mostly useless. You're bleaching the plastic back. The OxyClean is just to make you think you're doing something cool. Plus, once you add oxyclean in, I believe you cant even reseal what you didn't use, because it will foam up and blow up. Peroxide on it's own can blow up if it's concentrated enough.
Just go buy some 40vol cream developer from a beauty salon, and paint it on the card with a brush of some variety, and stick it in:A) The sun. Real sunlight is far more effective than a stupid UVB bulb like all the tards online will tell you to use for best results.. Kind of like how I stick my lizards outside in the summer and they get the required amount of UV in ~30-45 mins, as opposed to 8 hours under those expensive f*ckin' bulbs. The sun is free.
B) Stick it under a UVB bulb. Go get one from a petstore for like 10$. The tropical variety (5.0) is probably good enough.
I used to bleach old computers from the C= world in a fishtank full of 40vol peroxide that I stuck in my driveway. Then we upgraded to using a clear bag and a vacuum to minimize the required peroxide amount. In our case, we used the Clear Developer, instead of Cream, since we fully submerged the plastic.
You want to use the cream, since its thick and paint-on-able. Even a thin layer should do the trick. Theres a picture floating around of a yellow C128D that was done this way, and looked brand-new afterwards. Screw all that retrobrite crap.
Also, you can get this jug of crap called Urine Rescue. It's super-concentrated piss-remover. I used to mix it 50/50 with hosewater and got the same kind of bleaching results when Sally Beauty stopped letting me buy 10 jugs of developer at a time because all the teenywhores couldn't highlight their hair anymore.