SP's: 64 vs 96,
256 bit memory interface vs 320 bit
both are the same tech, so aside from the 512mb vs 320MB it's a significant downgrade.
They should have given you an 8800 GT or GTS, or a 9800 GT, not a 9600 card. Without doubt get in touch with them again and i'm sure they will replace it with something better.
The 9600GT is the slightly better card. It's not a downgrade.