^I was talking to the OP. But the Pentium D820 is a 64bit capable CPu:http://ark.intel.com/Product.aspx?id=27512&processor=820&spec-codes=SL88T,SL8CP
Intels 64bit started with the Pentium 4 5x1 series based on Prescott.
And to the OP, yes you need a 64bit OS to fully utilize all 4GB of RAM. Without it, you can only use 3.25GB to 3.5GB due to 32bits limitation of only 4GB total RAM (meaning system RAM, caches, GPU RAM) which all adds up. If you ahve a 1GB GPU then normally you get 3.25GB of system RAM. If you have a 64bit OS, you can support well more than 4GB (Something like 16,000 Terabytes of RAM) so it doesn't matter.