If you buy a good quality PSU (ie with good Amps on the V Rails) then a 250W would could run a P4 / 2GB / X800 256mb / 2 Sata drives / DVD-RW / mobo e.t.c
Tagan makes good quality PSU's, the Amps per Voltage Rail are more than generous, a 530W is not overkill, but if u buy it do not replace it for something else when u upgrade - u dont need to.
on a 530W Tagan PSU you could easily run:
Dual Core 4800+
2GB Memory (4x512mb)
2 500GB SATA-II HDD
2 x Optical Drives
2 x 6800GT in SLI
just have ot make sure you share your PSU V lines equally around your PC.