It's about 10% slower - about 2500 3DMarks. It only matters because I blew £1000 on graphics cards in September 2007 and now I have a slower system as a result of one dying and the manufacturer doing the right thing in sending me a MUCH better card, it just happens to be incompatible with my old SLi setup.
3DMarks, shmedeemarks, most games will have a less than a 10% difference and I doubt you could tell the difference with the naked eye. There are plenty of benefits to running single card anyway - less power usage, less driver and compatibility issues.
As you said, it's not even your main gaming comp anyway. Just sell the left over 8800 and pocket yourself some free money.