If somebody wanted to do some serious "crunching", and were on a budget:
Newegg.com has nVidia GTX460 video cards down as low as $160. These are very high performance CUDA cards intended for gaming. You need to have a powersupply of at least 400w and a PCI Express slot (normal expansion slot on machines sold in the last 5 years). And a mid sized case, not a "low profile".
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127519
Now, if your machine is a low profile, and a smaller powersupply, something like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500177 $80
or this
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121347 $50 after rebate
The nVidia CUDA cards are way more powerful than even an i7 quad core chip as far as computing power goes.
These also fit in larger cases, and play games pretty good, and stream video faster than you download it.
I love AMD and ATi products, but there is a simple fact of life: CUDA is currently a better technology for number crunching, and nVidia drivers are simply more powerful for most uses. ATI has the muscle, but their drivers aren't the best.