I have Archos 605 Wifi(80G w/DRV & mini-dock) and N800.

As many have suggested already, 605's strenght is in PMP(but the device is basically lock-up and the chance of any 3rd party hack is close to nill(unless you want to stay with the old firmware where there was a hack available). As for N8xx, PMP functions are not as polished; but it is not that bad(or un-watchable) and it is capable of doing a lot of other task well.

As far as media quality, it comes down to what is your definition of 'well'. If at all possible, you can try to get a hold on a demo N8x0 and test drive it with a few clips from your collection of video/music. This will give you some idea. Alternative, there are two video conversion program specifically for converting video to play well on N80x(one is from Nokia and the other is from someone else, please do a search in here and you can find it), You can use these tools to convert some of your clip and try to play it on your 605(yes, I do mean 605 here), this will help you to decide whether the visual quality loss of the coverted clip is acceptable for your usage or not.

good luck,