My Pc help info

I got this crappy computer which is:

AMD athlon 64X2 Dual core processor 2.90GHz

2gb ram

320gb

Window Vista

256 ATI Radeon HD 2400 pro

I can play fallout 3 but on sh*t graphic, i want to get the 9800gt, But my sh*t PSU is on 250W. Thats screwed up.

Now im wondering if i can even change my PSU coz the one i got is small, ones i have looked up are either to big for my pc or

won't fit in size, Is there anything i can do about it. Im willing to buy new psu aswell as graphic card, but i read forums who have the same computer as

i saying they cant upgrade anything from this pc, coz of its size as well as is dell, something like that.

Can someone help me what im suppose to do, dnt say my a new PC

um i dnt knw if it atx, what happens if it atx???wardogz007