Dell Optiplex GX280 - Small Mini-Tower Computer & 250W PSU power loading

Please can anyone advise about the Dell Optiplex GX280 Small Mini-Tower Computer, and how many additional internal peripherals can be installed and powered by the PSU rated at 250W?

I am fitting a graphics card rated at 65W, and have done calculations to rate the power consumption of the already installed peripherals and sub-systems, such as the SATA drive, combo CD/DVD writer drive, and all other onboard electronics, to determine if my PSU can source the extra 65W. 

Online PSU calculators consider all components running at a (almost maximum) power loading, and advise on the size of PSU required for a particular configuration of peripherals, although their figures and recommendations vary wildly.  Since all components don't run at 100 percent power consumption, some calculators use 85% or 75% of maximum load as a guide for the amount of peripherals the PSU can support, and some calculators also take into consideration capacitor aging in the PSU.  So it's hard to get a clear answer how many additional peripherals the 250w PSU can support.  I used the PSU calculator at http://extreme.outervision.com/

After taking all peripherals into account, including the added 65W graphics card, and a system load of 90% and 15% capacitor aging, AND then removing the DVD writer rated at about 25-30W and the floppy drive rated at 5W, the calculator advised a minimum PSU wattage of 231W and a recommended PSU wattage of 281W.

How accurate is this figure, and will the 250W PSU supply enough power?  Is it necessary to disconnect the DVD writer and floppy drive to be sure the PSU can handle the graphics card, and would this still be enough?

I note that many users of this machine install an additional hard drive and an extra CD drive, and probably a PCI card or two.  Surely this would overload the PSU according to the online PSU calculator, yet users seem to get away with this.

According to the PSU calculator, the GX280's PSU would only be capable of supplying enough additional power for only an extra hard drive, which sounds questionable.

Please can any informed users advise if I can keep the DVD writer connected as well as using the graphics card, or if disconnecting peripherals such as the DVD writer is necessary?

Many thanks for any time and help offered.

John