|DDR2 533 MHz is what you can use. if you put in DDR2 800MHz and it runs, it will underclock to 533 anyway (if it runs).|
Also note that your system supports only FSB1066 which is exactly in tune with DDR2 533 memory anyway. Since both are running at a natural frequency of 266 (DDR2 is a multiplier of 2 and FSB is a multiplier of 4), you are running a CPU/RAM Ratio of 1:1 which is precisely what you should be looking for.
Two of these kits should do it:
Since your system came with Vista 32bit and you list Windows 64 (which does not exist, I assume you have installed or are about to install Windows 7 64bit which is why you want 4GB or memory. Just remember that the system is pretty old and probably pretty slow. Installing 64bit Windows 7 will not speed things up and may just slow things down a bit. Do not invest too much money into upgrading that machine, save it for something new and significantly faster.
You have to be a little bit crazy to keep you from going insane.