1: Make sure target demographic is likely to have higher bandwidth. For example a website geared toward rural living clients should not have video, sounds and large graphics, simply because more rural areas statistically have lesser bandwidth.
2: Flash. In moderation and used for specific purposes such as delivering video content is good and most computers come off the line with the latest "Flash Player" installed. Flash is installed in some form on roughly 95% of all computers. Just beware not to make too much of the site out of flash. It simply isn't an efficient way of delivering content especially if that content changes often. Never ever use flash for the navigation of a site. Ever.
3: Quicktime. This is more specifically for video however Quicktime has (in my opinion) a better delivery method, has better quality and all around a better product, than most. While Windows Media Player may have a greater market saturation it seems to have inherent problems with it's ability to stream content smoothly on computers that are not huge workhorses.
4: As far as imagery is concerned stick with the tried and true: Jpg and gif. While .png's are gaining in popularity and can do some pretty neat things that gifs and jpegs can't, there still are some severe compatibility issues. *raises eyebrow in disdain of Microsoft* ... Although this will be resolved in the next one or two iterations of browser technology, it still remains a compatibility issue.
5: Finally... CSS. Never do with anything with an image that you can do with quality CSS.
Hope this answers what you were looking for.