Not really - until you have programs that refuse to run on anything but a 64 bit kernel or programs that need to allocate more RAM than they can in 32 bit address space you can keep running 32 bit on the Mac OS.
Running a 32bit OS instance does not preclude you from running 64bit applications. It's one of the more unusual things about Mac OS X, and very different to the Windows way of doing things where there are specific 32bit/64bit versions with fairly rigid limitations on what you can run.
As such, depending what you use your computer for there are some disadvantages to running in 32bit mode (addressable memory as per a different answer for one) but by and large you are not forced to run everything in 32bit just because your kernel is, and so the disadvantages are considerable less than for other OS.
See the following question/answer on Stack Overflow for more details:
You can have a play with 32 and 64 bit versions of the kernel by looking at the following instructions from Apple*:
If your Mac uses the 32-bit kernel by default, but supports the 64-bit kernel, you can start up using the 64-bit kernel by holding the
6 and 4 keys during startup.
If your Mac uses the 64-bit kernel by default, you can start up with the 32-bit kernel by holding the 3 and 2 keys during startup.
Your Mac will revert to the default kernel the next time you reboot it.
To select the 64-bit kernel for the current startup disk, use the following command in Terminal:
sudo systemsetup -setkernelbootarchitecture x86_64
To select the 32-bit kernel for the current startup disk, use the following command in Terminal:
sudo systemsetup -setkernelbootarchitecture i386
Note: This setting is stored in the /Library/Preferences/SystemConfiguration/com.apple.Boot.plist file and
will take effect every time you start up from this disk. If you start
up from a different disk, the setting on that disk, or the hardware
default, will take effect.
I am not sure why you talk about RAM in the middle of your question there, it doesn't seem particularly pertinent to me, and nor is there a specific question, so perhaps a question edit would be helpful to promote more answers.
(* Instructions for Snow Leopard, but should still apply)