Vista help

[burninhell]

i just bought vista ultimate (64 and 32) .whne i see my system configuration i see that 32 bit is installed. i want to install the 64 bit . i didnt get and options prior to installation whether to install 64 or 32 bit ??? any idea

my specs are

amd 3000
2 gb ram (kingston)
nvidia 8500gt
asus m2nmx