If you are strongly wedded to the PC world, then the Xeon processors are the fastest. However, they are not cheap. If you are open to the Mac world, then a G5 system (my personal preference) is very nice.
Lots of memory is the most important thing -- at least 1 GB, preferably 2 GB. Lots of disk is next -- if you are doing much FMRI, you will accumulate a vast amount of data. Fortunately, disk space is almost free now. You don't really need a SCSI disk -- IDE/ATA disks are good enough for single user computers (SCSI shines with multiple users doing heavy disk I/O at the same time).
I hope you are aware that AFNI does not run under Windows!!!
bob cox