Have a winner for you! the Sapphire Radeon HD4650 Low Profile Card. Works great with 250 PSU and is pretty damn powerful for something of this nature. And it will fit in a slimline case with very little issues.
The new low profile cards coming out by ATI and Nvidia need atleast a 300 or 400 watt psu so the HD4650 is the only generation that will work with a 250 psu\Slime Line Case with very respectful game play results. Nvidia has a low profile card but its lacking in horse power compared to the ATI.