64 bit multiply-accumulate

I have been unsuccessfully looking for a simple 64 bit MAC device.
I need:
1) this DSP or micro to be on a PC104 card
2) Linux support (or enough docs to hack)

My algorithm is very simple:
int x[4000];      // an array of 32 bit integers
int xt;           // 32 bit integer
long sum=0;       // a 64 bit integer
int i;

for (i=0; i<4000; i++) {
  xt = x[i];       // in assembler this would be mem->register
  sum = sum + xt;  // 64bit accumulator plus 32bit register to 64bit
accumulator
}

Now I know technically a MAC op might want the following:
  sum = sum*1 + xt;

Is there anything easy and cheap out there that will do this?  I
thought MMX commands would do this, but I think the MMX ALU is
strictly 32 bit operands.