64-bit Windows will run on about any processor newer than Intel Pentium 4 (late-model Pentium 4's are 64-bit) or AMD Athlon XP.
I think you mean "4 Gbytes RAM" instead of 4 Mbytes. You wouldn't be able to run anything newer than Windows 95 with 4 Mbytes of RAM.
4 Gbytes should be enough for Windows 7 64-bit (minimum is 2 Gbytes), but if you can afford it and have the space on the motherboard, I'd recommend upgrading to 8 Gbytes.
Casey