They're very acceptable for posting on the internet and other similar places, and even 6x4 photos. However, in most cases the optics aren't as good as a 'proper' camera, so while the resolution is comparable, often the quality isn't quite as good.
The Sony cameras always get a better writeup than the Nokia ones. I can also vouch for Sony's 2MP model in the W800, W810, K750, and maybe something else (all the same camera in different phones).
The k800 has an even better camera, and the interface is largely the same, so I can tell you it'll be good. The resolution will be better than your old camera, but it tends to be harder to get any artistic effects etc. with the phone.