The Debian buildd stats show that, at least on Intel or AMD-based PC platforms, the coverage is pretty much identical. (The stats cover unstable rather than Wheezy.) You'll find some 32-bit-only packages, and some 64-bit-only packages, but they are few and far between.
Will a 32-bit linux OS work on modern desktops that are compatible with 64-bit? considers the performance aspects; on the same, 64-bit-capable system, 64-bit code is often faster than 32-bit code, not because of the size of the instructions, but because the instruction set is more capable (in particular it has more registers).
So if you go for 32-bit Debian, you shouldn't come across packages which you can't install, and you will save some memory, but you'll lose some performance. You can get the best of both worlds though by installing 32-bit Debian with a 64-bit kernel (the
amd64 kernel flavours which are available as 32-bit packages); that way you keep the memory savings, you get better performance in the kernel, and you can run 64-bit software using multiarch if necessary.