you get twice as many registers, and some, but not all data is twice as large. the end result seems to be about 20-30% faster than running in 32bit mode (basing this on osc-count in synths + genlop -t compiletimes). certain things do feel slower, namely firefox and KDE, which could be due all the pointers. the big showstopper is all the coolest audio apps ( i mean the ones that arent just mediocre kde/qt knockoffs of what MacOS had in 1993 ) are not 64bit clean. someone should fedex miller puckette and james mccartney and ge wang linux-loaded amd64 boxen and demand a fix! right now i'd go Turion X2. if you want all 3 of: mobile, 64bit, dual-core, only AMD has an answer for you..
