I can definitely appreciate davio's point of view, and alot of the retro games people make, in construct and elsewhere, look awesome, and some even sell next to the big boys(http://store.steampowered.com/app/70300/?snr=1_4_4__13). But I want to present the opposing view. I love HD visuals. It's why alot of people buy good graphics cards. If you have a million years to work on your game, ideally, you would have it be able to switch resolutions, with up to that 1980x1080 most common resolution from the Steam hardware survey. I'd say 1024x768 is a reasonable minimum, if you're wanting almost anyone to be able to play your game, but if you have the time, and the art ability, there's a major shortage of games that take advantage of high resolutions. Just be careful with the VRAM, without bones or some other clever distortion, it's difficult to maintain that size, and have decent animation at the same time, not just with construct, but any 2d engine
if you're going to create hd sized textures though. I think anyone with a pc that had to run at 800x600 or 1024x768 wouldn't be able to run it, unless you had lower res versions of the textures as well