Low Profile Graphics Card Upgrade

I have recently aquired a Dell 5150c that has a 128MB ATI Radeon? X600 SE graphics card. This model uses the smallest BTX variant (picoBTX), this means the components are crammed into a space of 310mm high, 91mm wide and 363mm deep. There’s very little room for upgrading with internal components. There are vacant PCI Express slots (x16 and x1), but only half-height cards will fit. I am trying to find a Video Card that will greatly improve the graphics performance for a gaming use but am running out of places to look!