Can you guys tell me what are system requirements for a game I've made in Construct Classic?
No, that's impossible to tell. It's completely game dependent. A Windows pc and a DirectX9 compatible graphics card are the minimum.
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Roughly said any CPU can run CC games unless you're doing some crazy Python things similar to what Lucid used to (an animation system with something like 1000 Python operations per tick per bone, or what the hell was all that about?).
If you want to come up with some indicative numbers on paper look at the VRAM usage, double that and round it up to the nearest power of two. Say, if your game uses 37 Mb VRAM your system requirement would be 64 Mb assuming half of it is reserved for the system (assuming the user has Vista or 7, which have DirectX windowing, XP doesn't).
Edit: clarified explanation a bit