Depends on what you want the PSU to function for? Surge protection or a considerable backup power time. If you need backup time, the larger the wattage or VA, the longer it will run your PC.
If you need it for surge protection a simple small PSU will do, I would suggest atleast a 600W or +/-900VA. Since your system does not consume anywhere near 600W under load, you could get away with a mere 400W.