I got a couple of these two-platter drives for replacing the existing boot drives in my Mac Pro and PCs.
Talk about bang for the buck... They are really really fast and cost only $99.99 with free S&H at Newegg, see
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136218&Tpk=WD6400AAKS.
They are faster than even my old WD 10,000 rpm Raptors, and this has also been confirmed by others.
The only drive faster than this one is the WD 10,000 rpm spanking new 300 GB VelociRaptor
WD3000GLFS, but that costs $299.99, i.e. three times as much for 53% less space.
After that you're looking at two of these in RAID0, for only six hundred American pesos plus tax for your boot drive...
I'll live with the 640GB drive for now, it really is a truly great drive at a great price to boot (no pun intended :O).