AFAIK the 850 is the only one with a substantially better camera than what you have already. The walkman ones are a lot less obese than the k800i though. Mate had an W880i, didn't like it, don't know why.
I have the 880i had it for about 6 months now and really like it. Its a nice piece of kit with the brushed metal front the screen is very good. I wanted it because of its thinness and the music player. The camera is only 2mp but is enough for what I want. The only possible downside is the size of the buttons they are small, if you are a serial texter this might bug you I am not and have got used to the buttons
I used to have the sony ericsson k750i, great phones, with an average camera 2mp. Depends on what features you are looking for, like taking pictures, surfing the web, or listening to a lot of music. If its music, i would definitely go for the w910, shame about the 2mp camera but you can't have everything. Or the iphone...but i personally think its a rip off, and some of the features are so bad, for example the 2g (which is slow compared to 3g) for the internet.
If you like your sony ericssons like most people do, i would opt for the k850 with a cracking 5mp camera. Same with the samsung g600, good phone but texting is a bit of a nightmare, buttons are a bit small if you have big fingers like myself, its a bit of a girly phone to be honest (but i wouldnt tell my friend that). Finally there is my phone which is the LGKU990 viewty, great camera 5mp, touch screen, and can store music, videos and pictures up to 2gb. I didnt get a memory card with the phone but you can purchase one quite cheap off ebay (about a tenner). The internet is quite fast because of 3g built in. Most phones have memory cards in them so i wouldnt worry about storage.