Geforce7900, power supply & Dell Optiplex

hi

I don't know much, so here are the facts, and after my questions if anyone can help.

I recently got an old optiplex 745 for a $100 from my university. it came with an ATI radeon 9200 64mb. no manual, so i am not sure what mobo it has.

I wanted to install a geforce 7900 256mb that I got for free, since it seems a little better. It recognized it and it's running but I had a window saying I should plug the power cable. there is a 6 pin flat

the video card box says the requirement are 400w and the card need a direct cable. the optiplex has a 305w power supply.

According to what I read if there is no cable, the card performs accordingly to the 15 or 20w that it gets from the PCI-e, and if I had it connected it would get 75w. and I see the 6 pin connector.

I don't know much, so after the obvious I am lost!

1) I don't know what kind of cable I need! does it go to the motherboard or the power supply. there is a couple unused cables/plugs coming from the power supply, one 5 pin (for another hdd) and one 4-pin (like for diskette drive). Is there some kind of adapter from one of those?

2) is 305w enough? It only has one optical drive and one 7200 hdd. so I guess its not using all the power.

3) do I have to get a 400w power supply. how do I know if the mobo supports it? CPU was $75 so I dont want to spend $50 in a power supply. If I do have to get one, any recommendation?

or should I just go back to the original radeon 9200 64mb?

thanks!
Flo