4850 power supply

I want to buy the HD4850 palit version which has a dual slot cooler. I was wondering whether my power supply would be good enough as buying the card along with power supply will be too costly. I have a Umax 450W power supply which came with the case. It has 25A on the single +12V rail.I also have an AMD phenom 9550, 250GB HDD, 2 case fans and a DVD writer. I don't want to OC the graphics card or the processor.

I don't how much my efficiency is.

The problem is I don't think that I can spend my money on a PSU at the moment. I am going to spend it all on the card.

I just want to know whether I can use the 4850 for some months and then upgrade the PSU.

What is the worst thing that is going to happen if the PSU cant handle the 4850?

You say your PSU came with your case. Chances are excellent that it is a POS and will NOT be able to reliably power a 4850, possibly dying in the process. When cheap PSUs croak, they usually take other parts with them. This PC Power & Cooling PSU, http://www.newegg.com/Product/Prod [...] 6817703017 , is a mere $50, and is 80+ certified. An Antec Earthwatts 430, http://www.newegg.com/Product/Prod [...] 6817371006, is $55. Either of those should be good; personally I'd buy the Antec because I have had only positive experiences with this specific product (and any Antec PSU I've bought over the last few years).
You would be much better served by dropping back to a 4830 if you have to in order to afford to replace your PSU.