+1 you cant seem to get a good sq based multimedia system. Ive used JVC in my last 2 cars and really liked them. I personally if you can would would for the 8xx series. Also try caraudiodirect you can usually get a bit of discount of them and postage isnt stupid at £8 or something.
BUT if your buying a new headunit purely to add quality to the bose system you might be disapointed. The source unit isnt going to be 100x better then the bose cd player. Espically since to you will probably keep using the bose amp. If your that disapointed in the bose it needs to come out, all of it.
+lots
I changed my HU first mainly for multimedia and it wasn't an amazing difference in SQ but I had satnav, proper iPod control etc etc then I changed all 4 speakers and amped then seperate to the Bose and WOW.... I still ran them with the Bose sub and attached amp until recently which was ok but now with that changed too it is much better still.
So basically change the HU for functionality, but be prepared to have to change speakers and amp them to get any huge difference in SQ
Great responses guys - much clearer now! I will get up some pics once its done, but for now I will most likely go for the HU (not to improve SQ, i just don't want it to get any worse) but to improve ipod etc, it cant be any worse than an ipod ---> cassette lead! haha