The advantage of going to a 64 bit CPU and OS (a 64 bit system) is more memory addressability and the ability to perform higher precision integer calculations faster. So far, this is still in the workstation arena, although in 1-2 years it will move down into the realm of high-end consumer PCs, and in 5 or more into the mainstream as memory amounts continue to increase. The disadvantage is cost. The advantage of a 32 or 64 bit CPU on a 32 bit OS (a 32 bit system) is (usually) lower cost much lower cost per unit speed. It’s even easier than the cost benefit analysis used to pick what speed of CPU to get: if you need the capabilities of a 64 bit system, then go for a 64 bit system. Otherwise, go 32. Currently, anyway, that’s the case, although soon thanks to Intel 64 bits will come down to the same price level as 32 bit parts.
Also, if you’re building a new PC, remember that by the end of the year there will be PCI-E for your expansion cards and SATA will be possibily the only system connection for drives, Serial Attached SCSI, iSCSI and FC will all become more available and et cetera.
Without a 64 bit OS, due out this winter but given MS’s current problems will probably pushed back to around this time next year, you arn’t using it as a 64 bit chip: a 32 chip will give you the same performance or better at lower cost. And, frankly, 64 bits will not be needed for games for some time at least, although Doom 3 may have support for it to increase the speed of the game, the effect will be minimal, especially given the extra cost of the chip.