Right at the top of ATI's current spread of GPUs is the X1900. Slotting in at the bottom of the X1900 range is the GT.
The X1900GT features 256MiB of memory (GDDR3, of course) clocked to 1.2GHz. The core ticks along at 575MHz, making it the slowest of the X1900s. There are also 12 fewer pixel shader processors (although the same number of vertex processors) as its brethren.
As consellation for the knock-back in speed, aside from a reduced price of course, the card is compatible with an X1900 Crossfire Edition card, should dual-GPU be a route you're looking into.
The same IQ and video features as the rest of the X1900s are present, including the 64-bit FP precision HDR and Avivo.
Finally, the card is a single-slot cooled solution, so it won't take up more space than is absolutely necessary, leaving other bits of your lovely motherboard free for the addition of yet more expansion cards. As proof, here are a couple of shots of the card for you.