In reality its not quite that straightforward.....
A 'normal' TV signal - NTSC or PAL is *ALWAYS* 575 or 625 horizontal lines. I am not talking HDTV or digital signals etc...
I presume everyone above is using TV-out from a gfx card. When you change the resolution around - 640x480 or 1280x1024 or whatever, the graphics card and drivers convert the picture to something that has 575ish lines and then encode it to PAL/NTSC and send it out. So you are not really getting a higher resolution on the TV, its just making everything smaller.
This is why text in Windows is MUCH easier to read on a TV when you are in 640x480 compared to 1280x1024.
However, most modern graphics cards - especially ATI - do an extremely good job of outputting good quality TV pictures, so just go with what looks good.
You will probably only run into restrictions of resolution when you are outputting to BOTH a TV and a PC Monitor at the same time - the max resolution will be lower here.