I did a good research on your PC and all the available GPU's and I think your best choice would be an ATI RADEON HD 5570 with GDDR5 to be safe. Other cards either consume more power, or are less powerful. If you can't find a GDDR5 version, then go for the GDDR3.
The more powerful and more power consuming card is 5670, but I would not risk that if I were you.
The less power consuming and very safe GPU is 5450, but it is less powerfull. Your choice
Well, unless you find a very very cheap 7700GS then you can go for it, but if you can't then better save your money to build a whole new system, since your PC is very old, it's wasting money to spend money to upgrade that system...