i believe any PCIe video card on the market today will handle this task with ease. as far as the video card is concerned, x-rays are basically 2d images, even when dealing with 3D MRI or CT scans. But a cheap PCIe or an AGP video card will perform noticably better than onboard graphics
i'm pretty sure you wont benefit from getting a workstation graphics card such as a Quadro - these are designed to perform well in architectural drawing tasks, (eg, vector rendering and the like) and are relatively expensive. (You may be using some specialised 3D image rendering software or the like that will benefiot from one but if you are then the software documentation will certainly recomend you get one)
if you are dealing with many or very high resolution x-ray images you will benefit from lots of RAM (up to 3GB on WinXP) and a powerful CPU. the real work in handling large amounts of data is done by the CPU.