What is the best video card for a Dell Studio 540s Slimline desktop!!??? (using existing PSU)?

Hi, ive got a Dell Studio 540S slim line desktop and the graphics card is less than desirable. I want to get a new card with a budget of £150. The only restriction i have is that i dont want to upgrade the PSU. What shall i get!?|||i would make sure you even have the slot for the video card 1st as many dells leave the slot blank and come with on-board video.