Is the Optima Vision M good for 1440p gaming?

It depends on the graphics card fitted. A mid-range card handles 1440p at high settings in most titles; check which GPU your configuration ships with.

Can I upgrade it later?

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

Does it come with Windows installed?

Prebuilt gaming PCs of this type normally ship with Windows installed and ready to set up. Confirm the operating system on the configuration you choose.

Is it better than building my own?

Building yourself usually gets you better parts for the same spend and full control over quality. The Optima Vision M saves you the assembly and setup time.