Differences:
DDR400 - 2.6v, CL3
DDR333 - 2.5v, CL2.5

Does your mobo support 2.6v memory (it probably does, but you need to be sure). The problem here is that the Asrock board only has 3 settings, low, high and auto. You can bet that since there aren't any other options then the auto only switches between low and high. That should be fine for 2.5v memory (low will certainly be 2.5v), but there's nothing to indicate what high actually is, nor when it switches from low to high if set to auto.

The other difference is the CAS Latency, but that's relative to the clock rate. If you recalculate that as actual elapsed time you get:
CL3 @ DDR400 = 3 / (200 * 10^6) = 15ns
CL2.5 @ DDR333 = 2.5 / (167 * 10^6) = 15ns

As you can see it is exactly the same in terms of actual time. With your FSB it will only handle DDR333 rate maximum, so the SPD chip would probably compensate and allow DDR400 memory to use CL2.5 if running as DDR333, in which case there would be no benefit. However, if the SPD chip does not compensate then the DDR400 memory running as DDR333 would actually perform worse. You could probably regain the performance by setting the timings manually, but would you know what all the settings needed to be?

The DDR400 memory will probably work fine, but since there is no benefit and may even perform worse I don't see the point in taking the risk.