If you need help filling the blanks on your pc hardware specs, such programs like Everest Home or SIW can help you out. Just google em..
Then use a PSU calculator such as
this and like tedster says, add the headroom for comfort..
If you dont have one of those "Warranty Void if broken" stickers covering the screws/back of the pc case type of thing then it should be ok to DIY. Try and find the noise/sound (dB) rating for the product. Less than 18dB is pretty much as silent as one can get, 20-27dB is still quiet untill you get about 1 meter to it, anything above that is noise that can cause insanity with prolonged use
Installation instructions:
1. Power off the pc, unplugg the wall outlet, ground yourself
2. Unscrew the opening side of your case (screws usualy located at the back), slide the case open
3. Locate 4 screws that hold the PSU onto the case, and unscrew them
4. Remove the PSU, taking care not to damage any other components etc. You may need to remove a few PCI/Video/etc cards or cables to get it out
5. Install your new one, fit the screws back on
6. Trace the cables from the old PSU to the components its connected to, and replace it with the same type plug from your new PSU.
7. Clean up the cables, use the velcro (if supplied) or a cable tie to hold them together, so airflow to cool the pc is maximised as much as possible.
8. Double check everything you've removed have been put back in again (cables, cards etc), then pack up (which is just putting back your pc together etc..)
9. Dont forget to turn the switch on at the back of the psu.. it happend to me once.. '
