To fully see the benfits of moving to 64 bit, you'll need to upgrade Windows to 64bits, wait for all the component manufacturers to upgrade their drivers to 64 bit version and finally wait for applications to come along that actually take advantage of the feature.
So you're not going to see any advantage in getting a 64bit processor right now. Though there is a 64bit version of Far Cry.
But wait a second....it only works with AMD64.
My recommendation, go ahead and order a nice fast 3.8Ghz P4. You won't regret it. I didn't.
Remember also, that while hyper-threading has been around for some time, there are still only a handful of apps that use it. Any hardware changes that require coders to change their code to use it (like HT and 64bit) has always taken ages to go mainstream. Where coders don't have to program to see improvements (more gigahertz) you'll notice the difference.
I do a lot of video editing and graphics work and have 2gigs of RAM in my machine and even when doing crazy shit in After Effects, I still amn't using it all. The key reason for upgrading to 64bit is the availability of >4gigs memory. Who needs that?
I know there's 2mb of L2 cache, but many of the reviews I have read show no performance increase and some have even shown a decrease! (Comparing regular 3.8P4 to 64bit 3.8P4, 570J V. 670)
The other "benefit" from the 600 series (64bit) is enhanced intel speedstep technology. The 7700 is NOT a machine that was ever meant to run off a battery, so who gives a damn about power savings?
I had looked into getting a 670 processor in my 7700 when I ordered, but reasoned it out, and came to the above conclusions. Hope it helps.