Get a corsair, 520W.. its not ridiculously expensive. Its a quality piece of kit. It'll power your system no troubles and would power a slightly more powerful computer for when you upgrade. Assuming you will upgrade once, extending the life of your computer by another year or so, the PSU will be able to keep up and wont have degraded too much.
Remember, you will probably go for an entirely new system in 4 years or so.