Yeah, in fact I think it mightr even be possible to tighen the timing.
The board will not allow you to set the ram higher than a certain speed unless the fsb is high enough; and the fsb cannot be raised without overclocking the processor. If your processor is a Northwood (probably is, use CPU-Z to check) then you should be able to increase the FSB to at least 133MHz (533) and get a ram speed to 266 (x2 of 133 because its Double Data Rate). Note that there is absolutely no point in setting the ram faster than the fsb because the processor will not even be able to access the memory that fast due to the fsb limit.

Some equations to consider

processor speed = fsb x processor multiplier

fsb transfer rate = fsb x4 (because the data transfer is quad pumped)

so if the ram is running at the fsb speed
ram transfer rate = fsb x2 (double data rate) x2(if dual channel, which you should be since you're using a 865 chipset and have a pair of ram)