you can jump start the current psu by using a paper clip and connecting the green wire and a black one next to it.
just disconnect the mobo connector (see pic) and connect the 2, if it powers on, it might still be good. but if the light turns off when u hit the button there might be a short somewhere making the fuse go, and when you unplug it, it resets(maybe). for the dell psu, make sure all the wires look the same and go to the same places on the connector. if they do, try it, if they dont, then dont try it.
EDIT: whoa, if ur psu wattage is that low, what parts u got in the system? how many hdd's/cd/fans/etc...u got?
EDIT: n/m read ur post again, yea the system is underpowered a lil, 2 hdd's, 2 cd/s + all the stuff for the mobo/cpu etc. the one thing about oem psu's is that they are designed with just enough wattage tu run the stuff that comew with it, usually one hdd, and a couple cd's.