It's a bit pointless going over 800x600 cause then you're going over the natural resolution of a TV set. Anything higher makes things hard to read or see properly.
The amount of detail conveyed is exactly the same for all TV's designed for the same format. This is considered to be 720 x 486 pixels for the PAL format. NTSC is 648 x 486 pixels. Digital film uses 2048 x 1536 pixels (for those interested)
A TV is quite a low resolution display device especially when compared to todays PC monitors.
As for WinUaeX and achieving the perfect look - I've got close but haven't managed it yet!