Is the Sky Two worth it over building my own PC?

If you value a single warranty and zero assembly time, yes. Building yourself usually gets more performance per dollar and exact control over every part.

What graphics card does it use?

It ships with a dedicated graphics card, which is what separates it from office desktops with integrated graphics. Confirm the exact model before ordering, since prebuilt configurations can change.

Can I upgrade it later?

It uses standard desktop components, so memory, storage and the graphics card can generally be swapped. Check the power supply wattage and case clearance before planning a big GPU upgrade.

Does it come with Windows installed?

Prebuilt gaming desktops of this type normally ship with Windows preinstalled and drivers set up. Confirm the edition with the seller before ordering.

Is it good for 1440p gaming?

That depends on the graphics card in the configuration. A mid-range card handles 1440p at high settings in most titles; check the specific GPU before buying.