there are several technical discussions in this apparently simple question, and there is some historical and performance detail:

digital video on a computer has always always had to trade image resolution and quality against performance.
where audio playback reached close to perfection 10 years ago, the demands of doing video on computers, have not yet reached the same degree of ease.

for example doing uncompressed 16bit stereo audio takes 600MB/hour, doing the same in uncompressed video takes 60GB/hour at standard definition.
Doing video is 100 times more intensive than doing audio.

and at higher resolution than 720x480 the demands are still largely too great for most systems.

What has happened over the last 10 years, in digital video, is that the various subsystems required to do video have slowly increased in capacity - but not all at the same time, or at the same speed.
Disc speed, computer speed, internal architecture speed, graphics speed, all play a role.

What we have to do in digital video is a compromise- because doing things close to perfection is still too hard for all the sub-systems - or just too expensive.
So compromises are made on image size or compression of the image, to make the whole thing work.

These compromises mean that doing digital video can sometimes be quite difficult, as the person creating the content needs to have some idea of the capacity of the system that will be used to deliver the content.

What you have to do is understand and work within the capacity of the system you are using, and at this time ( early 2005 ), the best way to get the most - at most times - for an affordable cost - though not always - is to render at 720x480 or 720x576 and use a codec that compresses the ammount of information in the file like the dv-pal or dv-ntsc codec.

This gives usable performance on most systems.

Your screen might have a resolution of 1024x768 or even 1280x1024 but
going to higher resolutions increases the demands on the system to such an extent that other compromises have to be made - or lots more money needs to be spent.