How much of a power supply you need for your system depends on what you are using (your hardware/software) and how/what you are using it for (software/hardware).
Each type of activity can require much different loads on your PS (startup, idle, gaming, drive activity, etc.).
PS are commonly rated (most) for the maximum output they can provide. Sometimes you can get info about power ratings for specific voltages (e.g. 12v & 3.3v).
Many will design systems to match case type, since this can determine your potential maximum use. For example, the following is commonly used "rule of thumb." - MiniTower=350w, MidTower=380w, FullTower=450w and ServerTower=550w. These usually include at least a 30% margin of safety.
I believe you say you are using a 230 watt PS with a P4 system, 1 GB ram, 2 DVDs, 2 large hard drives & a graphics board?
I guess my question is not why it doesn't work now, but how it ever worked in the first place! I can only imagine you have changed the use of the system (gaming) or added additional hardware.
Hope you follow the above advice and replace the PS. Your system can suffer a lot worst than occassional blank screens.
Dell