My personal view is that 800x600 is still an important consideration:
1. Not everybody has their browser window maximised, some people prefer to run it in a window so they can see other desktop elements - after all it is a windows desktop operating system (and they are paying good money to Bill Gates for this feature)
2. Others might have other elements on screen or in browser (e.g. favourites column, search column, media column ) which reduces available screen width
3. Generally, having wide columns of text makes it difficult to read the text, as the eye has to travel further from the end of the right line to the beginning of the next left line - which is why newspapers have columns. If you just make your text lines wider to fill up screen width, without introducing columns, you will make the text more difficult to read
4. Although desktop PCs are getting higher resolution and they are the main way of browsing the net at the moment, this does not mean that will always be the case. There are already important inroads being made with alternative browsing technologies, e.g. mobiles phones getting better (and they have some catching up to do resolution wise, e.g. XDA aren't quite up to displaying 800 wide websites yet), Nokia 770 internet tablet is 800 wide, Apple iPhone, nintendo wii allows you to browse the web on your TV with its internet channel (which has the Opera browser with flash) and 800 wide is a good choice for this particular platform. So keeping a web design which supports 800x600 (whether fixed or fluid) doesn't necessarily give you a legacy web design, it can give you a more future proof web design.
A lot can be down to personal taste, a lot can be down to what you are used to (the designs of the websites you frequent), and a lot can be down to weighing up all available pros and cons. That is why you get different people saying different things, and different websites adopting different types of designs. Some go for the fixed 800 wide design, e.g. bbc.co.uk, lastminute.com (only ads go over the 800 mark), and most of our sitebuilder web designs. Equally, others go for fluid designs but still managing to squeeze down to 800 wide, e.g. amazon.co.uk, google.co.uk. There are also those sites that forego 800 wide and have fixed 1024 wide design, e.g. microsoft.com new home page. There are others that mix and match, e.g. w3schools.com is mainly around 800 fixed width and some of its other pages are fluid shrinking down to 800, ebay.co.uk has a fixed 800 wide home page, and other fluid pages that don't always shrink down to 800!
Of course that doesn't mean fluid is bad - it can work very well for ecommerce shops etc where there are a large quantity of products.
