There are after market head units with cassets, aux inputs (for plugging any mp3 player in, including ipod), usb ports, cd players, etc. Crutchfield has a wiring harness for aftermarket head units and harness adapters for the speakers.
With these you won't have to splice wires and connect terminals etc. (which is good if your not very good with electrical things)
If your interested, I have some stuff I could sell you pretty cheap.
I originally planned on just getting a headunit to play my ipod on and upgrading the speakers. But since then, I've gained and interest in audio and wiring etc.
Long story short, I now have 2 12'' subs, a 1200w amp, a 300w amp, and JL 6x9 evolutions.
Originally I had wiring harnesses for an easy plug and play install (for speakers and head unit) as well as a dash kit. I ordered from crutchfield so I have everything they offer. I wasn't planning on getting an amp, so I got speakers that run well on headunit-power. Infinity 6288cf I think I'll have to check the model number. But they're 2 ohm so they require less power for more output. Crutchfield recommended the setup.
PM me if your interested.
And as for keeping your cassette, it is possible to splice the original wires and put your stock headunit someplace and your aftermarket headunit someplace else. You could take the stock headunit appart and just keep the components vital to the cassette player. This would make it easier to mount or stash somewhere.
Where do you live?