Is this a HP machine?
You could run all that on a decent 300W PSU - as long as you didnt start any 3D gaming session.
If you started gaming the GPU power requirement could exceed what your PSU can provide.
Have you check the HP forums to see if any others with similar systems can are upgrading with 250GTS? That would save having to guess.
GTS 250 requirement from the EVGA website :
Minimum of a 450 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 24 Amps.)
so i will need atleast a 450 watt PSu? with all my other components ontop of that what would u think the minimum is? im upgrading my processor aswell from a 2.2ghz dual core . and yes the starting model was a HP ... ill check the website now thanks