The long and short of itis that 64-bit will use slightly more space (but not by a massive amount) for applications, will actually make use of >4GiB of memory if you upgrade, and will be able to use the extra 64-bit registers that were introduced which may improve performance by a couple of percentage points for applications some applications and by many percentage points for others.
If you ever even want to think about upgrading then go 64-bit.
64-bit will use more memory by requiring larger pointers for memory locations and code in programs, but I would not expect it to use a significant percentage more, modern programs are highly data orientated and that will be the same for both versions.
The fact that 64-bit would allow the OS to map device buffers and I/O out of the way of physical memory may give you more memory back though, it's all swings and roundabouts... I can test it a bit in a VM (can't dual boot my system atm), but I'll need a short while to check it out.
Also, there are the 64 to 32-bit compatibility libraries that are needed to allow both types of program to run at the same time and these will take an amount of memory though I'd be surprised if it was more than 100MB.
In summary if you are truly, honestly, painfully using every last bit of your 3GB of ram then 32-bit may be better as you could save a small amount of memory without all these extra bits that are needed for 64-bit.
Apologies for being a bit flippant, give me a day or two and I'll do some testing and get back to you.
I've done an install of both 32-bit and 64-bit versions of Arch Linux on Virtualbox, just to get a feel for how much extra RAM it uses for 64-bit mode. I followed exactly the same procedures for installing each and before getting each memory indication I did a restart so that the reported memory was clean "from boot".
Bare bones install, no GUI, just basic install there is about 26MiB difference (32-bit is about 60% the memory footprint of 64-bit) which is no surprise as it's almost entirely code and no real data:
X.org installed and running with the Virtualbox guest additions there is about 32MiB difference (32-bit is about 65% the memory footprint of 64-bit):
Then Gnome & GDM installed, no frills and running Epiphany there is about 80MiB difference (32-bit is about 75% the memory footprint of 64-bit):
So going up the scales of more code-orientated to more gui orientated we go from a 40% difference in size to a 25% difference in size. I would expect that difference to get smaller as you use more and more actual GUI type programs. There's also shared libraries to think about too, as more programs share the same code from a library they will not be using as much memory so the percentage size increase will be smaller still for a fully installed and ready to use system.
There is a definite memory hit taken for using 64-bit, but it will get smaller and smaller as you use programs that are storing and using data rather than actual hard code. The code for 64-bit Firefox will be a significant percentage larger than the 32-bit code, but the actual data stored to cache webpages between the two will be exactly the same.
I've not gone into how much you might save by the 64-bit kernel being able to remap devices out of the 32-bit address space, it's a bit more difficult to do in a virtual machine, but I personally would still take the hit and go for 64-bit.