Can you share with us what your upgrade will involve? Which CPU, how many drives, other peripherals, gadgets, fans, etc. Your requirement will be directly dependent on the components you'll have in the system.
Generally, with recent hardware, 250W is on the lower side and may not provide enough juice for rock-solid stability. However, I have a system with a 1.33ghz Athlon/512MB/60GB/CDRW/CD-Rom/NIC/Radeon7500/SoundCard/other junk that has been very stable for the past couple years (using Enlight 250W). But all of other machines have 350W or higher PSU (all Antec or Enermax).
Anyways, whatever your system requires, I would never cheap out on a psu. So tell us what your system is going to be and we can provide better recommendations.