Available today - yes
Mainstream - no
The present addressable memory limit with IA32 is 4 Gig. Once upon a time, a machine with 1 GB was unheard of; today it's commonplace. We are fast approaching the 4 Gig barrier.
iAMD-64 bumps the addressable memory limit from 4 Gig, to 256 TeraBytes. Let's just say that 256 TeraBytes of DDR is a really BIG jump forward.
Neglecting PCI-express and multiple Hyper-Transport links (ie. dual core 64 bit processors in a single package; that drops into your existing motherboard), this is where we are going. Nvidia and ATI have already released PCI-express Graphics engines, which bump performance upwards of 300% over existing technology.
Gamers will very likely be among the first to jump on the bandwagon. Next will be those who want to up-band their video through a graphics engine to their HDTV. iAMD64 will rock the world like Intel did with the Pentium against the x486.