Dimitris

All dual core (and above ofc) CPUs are able to run at x64.
Some people think that 64-bit systems require more from hardware so they can run slower on some CPUs. Simply - that is false.

1 GB is way too less for big mod packs like B-Team and the other. You should get more RAM.
It is possible to get more than 3.25 GB on 32-bit OS by using a modified kernel - but that might cause many severe problems. If you want to extend your memory to 4 GB, get a x64 OS.