I happen to be a computer engineer and a software programmer so I understand the very lowest levels of the hardware and how the software interfaces with it.
Here is the deal with Vista 64-bit dude:
First the negatives:
Microsoft has not yet changed most of their ISA (instruction set architecture) which means most of their instructions are still 32 bit. So in Vista 64-bit, the OS essentially wastes 32 bits most of the time. Microsoft will change this over time so that they are taking full advantage of all 64 bits.
So if you had an XP machine that you had 2 GB of RAM in and you liked how it performed you should put 4 GB in a Vista 64-bit machine (this does not account for the approximately 10% extra memory usage needed by all the new overhead in Vista)
Now for the positives:
If you give Vista 64-bit enough RAM (I put 8 GB in my Vista 64-bit machine) it will actually run a little faster than XP even with the overhead (while the 32-bit version of Vista is actually slower than XP). This is because of the differences in the way memory addressing is handled between the two. Vista 64 can make much larger jumps when fetching instructions or data without the penalty of extra clock cycles to calculate absolute addresses from indirect addresses and offsets because a 64 bit instruction can always contain the absolute address of data needed for the instruction.
Microsoft has the vast majority of the driver issues that existed at launch all worked out now so honestly there isn't a better gaming rig option than Vista 64-bit.
Also, Vista will be tweaked over time to improve performance just like XP has been since 2001 so it is only going to get better.