First the bad news: That PSU is just not enough, it will have to be upgraded to cope with even a limited graphics upgrade.
Now the good news: Your case seems to be a standard width, so most cards will fit, but you need to measure the distance between the back of the case and the front to see what length of card will actually fit. Also prices are beginning to fall, so you might bag a bargain if you shop wisely
Here is how to check the card length:
Get the photo from Newegg and paste it into a graphics progam like Paint. Cut out the PCI-E edge connector-this is 85 m.m long overall-and paste copies along the length of the card image. With a little math you can calculate the exact length of the card by scaling this way. Also look carefully at the case and motherboard, most powerful cards take up two slots of space, a problem for some.
While there is HD4870s for this price, they all have crappy coolers that dump their heat into the case or have loud fans, or both:http://www.newegg.com/Product/Product.aspx?Item=N82E16814161244
And the PSU:http://www.newegg.com/Product/Product.aspx?Item=N82E16817341016
Others may suggest other components, but the PSU is a steal, especially if you get the rebate;)