64 bit games are optimised to run on a complete 64 bit platform i.e. a 64 bit processor with a 64 bit OS. (For e.g. Athlon 64 in true 64 bit mode with a Windows XP 64 bit edition). The 64 bit platform enables more data transfer / enormous computing power and thus enables the games to be run at mind boggling quality.
Actually, the Graphics processing Unit (GPU) on a graphics card has a core of 256 bit compared to 32 bit for a normal processor. So during data communication and processing the CPU always acts as a bottleneck in gaming. in 64 bit computing this bottleneck is eliminated 2 sum xtent thus enabling more processing capabilities.
Far Cry 64 bit is already out. UT2004 64 bit edition is goin to be out. Many othr games will soon be out thr for true 64 bit gaming.