i did some runs using superpi v1.4.
i used different multi's on a unlocked xp2800 on my msi delta. i used 1gig stick of corsair 3200pro with the fsb set to 166 and timing at 7-3-3-3 to force it to single channel.
11x166= spi/58.7 seconds
11.5 = 57.1
12 = 55.4
12.5 = 54.5
i have a xp2400 setup in a soyo kt600 ultra plt with 2x512 sticks of corsair 3200c2pro. i set the timing again to 166 with 7-3-3-3 timings. the tbred has the 256 cache which should be what you have.
11x166= spi/64.8 seconds
i don't no how accurate this comparison is but it should give you and idea. as you can see by the scores the gain drops as the multi goes up. haven't a clue. no i didn't feel like playing with the dip switch to set the other multi's.
run superpi 1.4 on your present system and see what you get.
http://superpi.radeonx.com/
ud