[AVON] I'm using 800x600 in C# / XNA for compatibility with the XBOX 360, but would not bother with any less than 1024x768 for PC.
[.ALEX] doesn't make sense...I've programmed pretty extensively in C#/XNA and XBOX360 can do plenty of high resolutions. 800X600 isn't even 720p.
I didn't say 800x600 was 720p. I'll let someone else explain my decision; the following text is pasted from http://xnaresources.com/pages.asp?pageid=27
About Resolution
We've included three different resolution images in this package, mostly for PC support. On the XBox 360, handling resolution is really dependant on what you want to do with the rest of your game. The XBox 360 will take your preferred backbuffer size and scale your game to fit the screen, no matter what display mode the user is using. I've decided to stick with 1280x720 for High-Def resolutions (720p) and 800x600 for Standard-Def resolutions. The 800x600 will scale automatically on the XBox to 480p and allows for a standard since that easily works between the XBox and the PC. 1024x768 should work just as well, but there will be more of a quality loss when scaled on the XBox.