Gateway and Dell

I have my moms old gateway it's been sitting around for like 3 or 4 months now so i figured i should upgrade my dell dimension 4600c slim profile the power supply is only 160 watts and i have upgraded everything but the motherboard, cd drive, and power supply and i figured i'm getting pretty close to burning out my power supply and i should get a bigger one but this power in my dell now is one of a kind this is the only one that will fit in my dell. i'm planning on first getting a zalman heatsink to replace the one i currently have just because i want some led problem is i only have one 4 pin molex connector and thats for the hard drive second the motherboard only supports dell fans so i was going to buy a splitter and a 3 pin to 4 pin adapter for the fan but instead i thought well i will just take the PSU out of the computer i'v had sitting around and just lay it on the desk next to my computer no problem and them just buy the adapter to lower costs. do you think that i would be ok to swap them out. i checked the wires to make sure they are the same 3 times and they are the same. the power i want to use is a astec ATX250-3505 and the one in my dell is stock from a dell dimension 4600c. any opinion would be greatly appreciated. Thanks. :confused