cheaper the better
is not a good thing as far as PSUs are concerned a PSU failure can take out motherboard, cpu, memory as well as hard drives, i.e. itwi;; cost you a new PC and possibly all your data as well.
A Corsair 400W PSU may seem small, I would go for at least 500W, it will have no problem powering a PC that requires 200-300 watts of steady current:
If renewing a PSU check:
The physical size of your PSU, some are hard to replace due to being a non standard size.
The amount of power need from the PSU don't skimp.
The correct connections for your equipment
1. Physical Dimensions
Besides the specs and form factors, the physical dimensions are also important factors in selecting a compatible power supply. Here is an outline of the physical dimensions of most standard power supplies:
ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.
Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.
MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.
Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.
Use the data above to determine if a particular power supply would fit your case.
The quality of a power supply can be estimated by its weight. While this is not a true scientific or thorough measurement of the power supply reliability, it is nevertheless a very simple and easy way for ordinary PC users to estimate and compare the quality of a power supply. Why weight matters http://www.directron.com/psu.html
2. Power supply calculator http://www.antec.outervision.com/
3. Correct connections Some boards have 20 pin connectors others 24 pin There is often a 4 pin plug required to power Intel CPUs Molex D plugs for IDE HDD and CD/DVD drives SATA power connections for latest HDDs and DVD drives.
Guide to changing PSU http://www.videojug.com/film/how-to-install-or-change-my-computers-power-supply