That is what one of the T61 manual says. That 3GB is the max ram on the T6x series.
Certainly confusing. Allow me to clarify based on my direct experience (can provide screen shots to prove...) with T60 and T61 Thinkpads.
The T60 can not support more than 3GB of RAM in ANY situation, 32 or 64-bit. That's a limitation of the "Napa"/945 chipset. You can install 4GB of RAM in these machines, and the BIOS will claim to "see" it all, but only the first 3GB will be addressable/usable by the operating system. Some argue that it makes sense to do this as you get synchronous double-pumping by having two identical modules which does increase speed somewhat... The chipset limitation here makes running a 64-bit OS largely moot, though the 945 chipset does support the Core 2 CPU which has the appropriate x86-64 extensions which would allow running a 64-bit OS.
As George notes, this information is out of date. The "Santa Rosa"/965 chipset which the T61 uses absolutely can address 8GB of ram. This is how I have my T61p configured. 64-bit Windows certainly does see, address, and use all 8GB of ram. I have 32-bit XP on another hard drive which I use for games, and it only sees 3GB.
I'm a noob so sorry for asking when I ask what would be the perfect 64-bit OS set up on the T61?
4GB is *absolutely fine* for the vast majority of purposes. I've found 64-bit W7 to be more stable than 32-bit, generally speaking, so I'd say 64-bit and 4GB of ram is "perfect" for the vast majority of people performing the vast majority of tasks on a T61.
I currently have WinXP Pro on the T61 so by upgrading to a 64-bit OS I can benefit from having 8MB of ram installed?
Sure--but unless you are doing things which really need more than 4GB of RAM, you're just flushing money down the toilet. 4GB DDR2 sticks aren't exactly cheap... Check out task manager and monitor your RAM utilization. OTOH if you're the sort of person who likes to drive your Lamborghini Murcielago to 7-Eleven to pick up a Kit Kat bar or to go through the McDonalds drive through, 8GB of RAM to do word processing, web browsing, and movie-watching/music listening is DEFINITELY the configuration for you!
My T61p leads a dual life--I have 64-bit W7 on a SSD, which absolutely smokes and allows me to do have a million things open at once across dual displays and have virtual machines flying and everything is great. Then I have 32-bit Windows XP on a 320GB 7200rpm Western Digital Scorpio Black hard drive, which I use for occasional gaming. This only sees 3GB. I often end up doing light web browsing in XP and there is pretty much no difference compared to the W7/SSD setup.
For someone who likes 4:3 screens, your money is best blown on a T60/T61 hybrid wherein a T61 motherboard is installed in a 15" T60 chassis so you can use the fantastic 1400x1050 or 1600x1200 FlexView IPS displays. Such a configuration (ideally with Intel, not NVidia graphics, to avoid the whole "flaky NVidia chip" issue) is unquestionably the ULTIMATE 4:3 machine.
If you have 4GB of RAM now, and are set on pouring money into this T61, consider finding a T9300 "Penryn" processor -- these do make things a little zipper, as well as running cooler than their older Merom counterparts. Battery life is also improved. These drop in and run fine on early T61s using Middleton's BIOS.