Actually folding@Home acts the same way in 32 and 64 bit systems in terms of helping. The standards are 32 bit calculations. The hardware and the software of Linux 64 Bit Systems act like an 32 bit anyway.
What will really help is when you Buy an Nvidia 8800 with Phisics enabled processors, they will do the same thing as your processor, but with lower overheads causing better performances even with lower clocks.
Common desktop processors are not equal Cell SPEs or phisics processors, they have to work with kernel overheads all of the time to secure redundancy of the system components and the threads you get to them.
In the case of common processors, the FPU at higher clocks speeds help more, Pentium 4 benefits the process.
Helping folding@home can be done in two ways:
Higher clock speed processors or
Lower overhead/latency to calculate/More cores