im thinking that you should be able to handle that GFX card on the stock 375w PSU, though, u still might want to get a new PSU, you would only need about a 400/450 at most of a good brand, eg Corsair, PC Power and Cooling
I heard this card is quite large and may take up two slots. Will this be OK in the Dimension 9150? I have looked at the docs (See Below) and there seems to be a slot next to the PCI-E slot that is not being used. Will this card fit in to this area?
Antec ( EA500D ), Corsair ( VX 450 ) and PCP&C ( Silencer 500, doesn't need to be the Dell compatible version ) for psu upgrades if needed, don't know that you would find any of them at that low a price though.
Yes, the dual slot card will fit, someone on another forum even managed to stuff a GTX280 into the same chassis.
As far as I know the GTS 250 is just another rebranding of the 8800 GTS 512, so it should do well in most games.