So Nvidia launched this SDK for their 8 series graphics chipsets called CUDA (Compute Unified Device Architecture) around Feburary 2007.
It basically provides a more generic method for performing calculations using their GPUs. They have implemented a fairly simple low level api in C.
The SDK is forwards compatible so the real power of this will come in a year or two when many machines will come with Nvidia Series 8 hardware or greater.
I spent a bit of time perusing through the programming guide as well.
You can get some basic hardware from newegg for around 100 dollars (8600GT 512MB ram). Or if you are really excited you can get a 8800Ultra for around 800 bones.