int i, j;
//init
float x = ((float) i)/j;

ends up with the exactly same result on any computer within one CPU
architecture, one operating system and one compiler. I don't really care
what the exact number of the result is, i just need it to be exactly
the same on multiple computers with the same architecture(in particular i
ask about x86_64).

An example:

I have two computers. Both of them are have x64 Debian. One of them has
Intel C2D processor and the other one say AMD Phenom II. Now on C2D i
compile a source code stated above. The question is: Will the x variable
contain exactly the same result on both of the computers for every
possible i and j values(same on both computers of course)? Now, do not
consider any compiler optimization - let us say the value of the two
variables is not known during compile time... Again, I don't care about
the precision of the computation, i just need the values to be the same.

Then you should use your compiler options to specify IEEE accurate division (icc -prec-div or one of the options which implies it, such as -fp-model source). For gcc, you would avoid the reciprocal-math option. Accuracy of the approximate reciprocal instruction on AMD64 is less than it was on the earlier AMD32 CPUs, and both AMD versions differ from the Intel version.
Needless to say, in more realistic context, other issues may come in. We can't guess how you are using this.

If you use x87 code (the default for gcc 32-bit) you will need to make sure the precision mode is set the same in all cases (e.g. by controlfp()). You would avoid this and get the same code as the x64 default by setting an option which supports sse; e.g. -march=pentium4 -mfpmath=sse
In either case, as I mentioned before, you would avoid the reciprocal-math option.
If you have questions about gcc, the more appropriate place is the gcc-help mail list (sign up at gcc.gnu.org).

>>...I don't care about the precision of the computation, i just need the values to be the same...

Tim answered all your questions. I'd like to add that calls like:
...
uiControlWordx87 = CrtControl87( _RTFPU_CW_DEFAULT, _RTFPU_MCW_PC );
or
uiControlWordx87 = CrtControl87( _RTFPU_PC_24, _RTFPU_MCW_PC );
or
uiControlWordx87 = CrtControl87( _RTFPU_PC_53, _RTFPU_MCW_PC );
...
at the beginning of your processing will guarantee that FPU settings for all platforms and computers are consistent. Only in that case your results will be identical for some C/C++ compiler but could be different (!) with another C/C++ compiler.