The question to ask is: how big do you want to print? Wassat, you NEVER print? OK, then you need no more than a 2MP image, which will easily fill the average computer screen. Oh, hang on although your images are mainly posted on the web, you occasionally wanted to print A4?
The rule of thumb for commercial printing (magazines, books, etc) is that you need no more than 300 dots per inch (dpi) on the printed page. So for your A4 print, you need around 6 to 8MP, based on a 6MP image being about 3000 pixels wide (3000 pixels divided by 300dpi = 10 inches). That's it, job done!
For detailed comparisons of digital cameras, see here.