and enter all your hardware, to find out how big a power supply you need. Then you will have to find out what your current PSU delivers. From experience, this is on a sticker on the PSU, but you will almost certainly have to open the PC case to see it, and you may well have to remove the PSU from the case. My PSU's sticker is on top, and not visable when the PSU is fitted.
Don't open the PSU though, for safty reasons.
I would recommend that you have a 'margin' of at least 50w between what the link above suggests, and what your PSU supplies.