ok... let me lay this out for ya...
32bit OSes allow 2^32 bytes (4GB) of physical memory to be addressed, 64bit OSes allow 2^64 addresses (17.2 billion gigabytes). If you install a device which has any physical memory onboard (eg video cards, sound cards, even HDD caches), windows must use some of these addresses to allow use of these expansion devices... therefore you will be able to use that amount less of 4GB of RAM on an x86 system. This probably won't amount to more than 600MB, leaving you 3.4GB to work with, which really is plenty. Plus, you can always upgrade to an x64 OS later on to fully utilise the RAM.
4GB is getting more and more necessary, especially for multitasking and switching large apps on and off (eg if you wanna play Crysis then switch to browsing the web quickly)... if you wanna do some more reading on this, there's an article here (http://www.corsair.com/_appnotes/AN8...e_Analysis.pdf
) that is quite informative, but kinda biased.
Also, interesting to note, that the larger int value employed in x64 OSes slightly increases the amount of RAM used on any given app, but it is quite insignificant (usually less the 2-3%).