This page is written by a non english speaker; please excuse the bad grammar (And correct it!).
GPGPU also know as GP² is an acronym for General Purpose Graphic Processor Unit.
This simply means the use of graphics processors -GPUs in place of CPUs- for general purpose programming.
This article talks about commodity hardware, the video cards that are toys to play games, sold to common PC users, and his capabilities as math coprocessor for CFD.
Graphics processors have many orders of magnitude more power than PC chips. They equals the power of little MIMD/SIMD clusters (up to 10 PCs CPUs) in 2005. But in some applications they can gain up to 50x the power of PCs chips.
A graphic unit have a main memory (up to 256/512 Mb by card in 2005), and a graphic processor with many stages and parallel processors; each stage trow his output data on the next stage.
First stage: Commonly is the vertex processor. It have commonly less parallel processors than fragment stage. The vertex processor does calculations specialized in matrix transformations. Typically project 3D coordinates in 2D screen coordinates, but is not restricted to matrix products. It can be thought of as a vector -or quaternion- processor, since a vertex is one or two vector of 4 components representing a triangle vertex. It adapt well to Lagrangian frames.
Middle stage: Then, follow a (still non programmable) stage where linear interpolations, and other works are done. This stage has poor flexibility, but can be useful to specialized tasks.
Third stage: The last stage, the more useful for CFD, is the fragment processor. Latest GPUs from 2005 have up to 24 parallel fragment processors, capable of process 4 IEEE floating point each one (24x4=96 parallel calculus!); here 2D matrix of four numbers are processed. They map to 2D screen pixels containing four numbers -Red, Green, Blue and Alpha components of pixels- Can be thought like a 4096x4096x4 matrix of 32 bits numbers/data.
Languages: Is possible to program directly with OpenGL Shading Language -GLSL-, his equivalent of Microsoft, DirectX shading language HLSL, or Nvidia CG [free CG toolkit]; all in a format very similar to c/c++. OpenGL and CG are full portable to non Microsoft environments. Those 3 languages are almost identical. Also exist languages like Brook GPU and c/c++ libraries/wrappings. Since representation of CFD data requires graphic drawing, learning OpenGl is extremely useful for CFD, and from here, programming GPUs is a very straight forward step to do.
To end of 2005 two manufacturers produce programmable video chips for PCs: ATI and Nvidia, but ATI have only 24 bits of precision meanwhile Nvidia reach 32 bits.
How much power? In a Geforce 7800 have been measured 160 Gigaflops (not peak, but maintained performance). But expect half to 1/3 this power in a general purpose/novice program. There are at least double chips video cards, and PC motherboards that support up to 4 video cards. This mean 160x2x4=1.2 Teraflops (1.2/2=600 Gigaflops) on one PC with 512*4=2 Gb of 'video' RAM. But nvidia drivers support transparently only up to 2 chips running like one and without the double of memory. Then, for the novice there are only (160/3)x2=100 Gflops and 512 Mb of video RAM available on 2005 at cost of near 1000 U$S. In comparison, there is possible to put 2 x86 processors with double CPU on a motherboard, allowing up to a peak of 15/20*4=60/80 Gflops in a PC, that can be reached by programs that not fill the cache.
To see an article on double chip video cards see this article: Two's Company, Four's a WOW! Sneak Preview of NVIDIA Quad GPU SLI, and for a quadruple PCI express motherboard see this: One Gigabyte Motherboard, Four Graphics Cards.
For more information see www.gpgpu.org
For a tutorial on 2D fluid simulation on GPGPU see this PDF: GPU Gems Fluid Chapter
For free books on OpenGL download the red, blue, and orange books from www.opengl.org