Ejector, a 1kw PSU will run happily with loads of UP TO 1000w. It'll only draw as much power as the system requires so if your PC needs 300w to run then that is all that will be used.
My motto is simple, always get the biggest PSU you can afford as, since you ain't going to run it at it's maximum load, there is less "stress" on the PSU and that means it should last a heck of a lot longer.