Well if you want to know for sure, you can always add up the total wattage from each device in your PC and give yourself another 20% headroom or something like that. That will give you how big of a power supply to get. 350watts is fine for what you are running now. It's a basic system and that power supply will run it ok. I mean, you are running on 250watt stock PSU so a 350watt would be a good increase.