windows 32bit (x86) has a max ram block built in by microsoft. mostly due to expected drivers issues.
have a look:

64bit linux is different as things work now. 32bit programmes simply use 32bit libraries as needed.

it used to be like you describe, but things change. they change quite fast in IT.

plenty computers though still ship with windows home basic or similar with max ram at 4GB. i just got a laptop with windows 7 starter and 2GB ram (which is the max win7 starter hcan handle). the CPU is 64 bit and the max ram on the laptop can go up to 8GB. so i guess they expect me to upgrade the OS. and i plan to - to upgrade to Ubuntu 64bit

it is also true that performance improvement might not be so big with less ram, but if you have 64bit CPU then why not use 64bit OS? and who knows maybe you would upgrade ram in near future and if you use LTS you wont' need a reinstall to go 64 bit.

anyway you can read a bit more here:
Which is Better - 32 or 64 Bits?

If you are doing heavy work where you have started to hit the 4GB memory barrier, then 64-bit is for you. Certain intensive tasks such as encoding video or audio also run significantly faster on 64-bit operating systems (NOTE: this is implementation specific).
Phoronix has done some testing (2009), comparing 32bit/PAE/64bit, and this seems to indicate that 64bit performs better than 32bit in almost all cases.
Early 64-bit adopters were plagued by incompatibility problems (most noticeably Java and Flash), however most issues have now been resolved.
Some applications such as Flash do run slower in 64-bit mode, however work continues to improve on this.
Other platforms which also come in 32 and 64-bit flavours may experience more problems especially due to a lack of 64-bit device drivers as incompatible user application. As Ubuntu is entirely open source, this is not the case as all hardware supported by Ubuntu works equally well in 32-bit and 64-bit environments. The same applies to open source user applications as well.

Which Should I Choose?

Unless you have specific reasons to choose 32-bit, we recommend 64-bit to utilise the full capacity of your hardware.
some people say even flash doesn't seem to be an issue anymore. but i guess ti still depends on hardware/drivers....