Which OS Level 32 or 64

How do I determine the OS Level I'm at?
I installed Vista Ultimate Home edition about a year ago, and forgot which
option/path I took.
Want to upgrade from the two 512 Meg Memory DIMs to Max possible.
Option is two 1 Gig or 2 Gig chips.

Need to know the OS Level .. is it 32 or 64 bit.

Hewlett-Packard Model DV9610 :HP Pavilion dv9000 (RU971UA#ABA)
CPU:AMD Turion(tm) 64 X2 Mobile Technology TL-52Memory:1.00 GB
Hard Disk Drives:
"C" - 22.08 GB Free (105.20 GB Total)
"D" - 326.44 MB Free (6.59 GB Total)
"J" - 458.06 GB Free (465.65 GB Total)