Surfing through the 7800 GT scores they go from 3800-5500 for "identical setups" to yours.
The non-sli are 3800-4000 then with SLI they jump to 4766-5525.
(4766) Low end guy's gpu clock is 468/1096. (A8N-SLI DELUX) 8.1.9.8 drivers
(5100) Mid range guy is 425/1099 (A8N-SLI) 8.1.9.8 drivers
(5525) Hi end guy is 425/1174 (Microstar MB) 8.1.9.8 drivers
Ram and CPU FSB and clock speed are all the same. There is no hardware reason the scores are that much different. It has to be background tasks or card settings. Your rig is benching on the low side.
Reboot, turn off everything non-essential and start the benchmark immediately. If the bench is still low email the guy with 5528 and ask him what he is doing right. Possibly a different sound card? Or he overclocked and 3dMark06 does not catch it. '05 and earlier used to show the cards real speed.
I occasional get an email about a benchmark result and actually answer them. Unfortunately, the ansewr is usually I have no clue what I did right.
UPdate - now that I wrote all that I see I was looking at 7800 not 7900. Ooops.
The 7900's do not appear to run as fast. Only 3 guys beat your score and they must be overclocked. Speed shows as N/A.
The typical score is in the 4400 range. So you are doing really good, quit whining. Unless you want to whine that the 7800 GT is a faster board.
It could be worse. My POS benches at 1700 and for what I paid I should have a score close to 4000.