Which Small Graphics Card for Micro-ATX Cube?

I'm building a small mATX PC with a cube case: http://www.ebuyer.com/product/122772
Comes with a 650W PSU for £49.99 so it's quite good value and has good reviews on it being silent and reliable.

It's going to be powered by an E8400 so the rest of the power goes to the graphics card.
Now I'm not sure how big a graphics card it can take but looking at the size of my existing X1900XT I can guess it won't be able to accomodate such a card.

What I'm looking for is a card that could possibly be more powerful than my old X1900XT (as building a new PC with a slower graphics cards is a bit pointless). However important factors include:

The size of the card (to be able to fit in a small cube)
The amount of heat it produces (small case = low airflow)
Overall power (I want to be able to play games on my 24 inch 1900x1200 monitor)

Any recommendations?

In fact I'm also open to recommendations for other mATX cases. (ebuyer offers very little choice)
Thanks!!

The 4670 is a rip-off for the price right now, only worth getting if you have a measly PSU (or if it's the only thing that will fit). You can get a 9600GT for the same price.

Though I'm not really sure the largest size card a mATX cube case can hold. The 4670 unfortunately may be your most powerful option that will fit. And if that's the case, you can forget about playing any recent games at 1920x1200 without turning some settings down quite a bit.