P5K Straps -By The_Stilt @XS

On Asus P5K-series there are two strap changes:

At 293FSB the board changes FSEL strap to 1333FSB to make more memory dividers available. This doesn´t have any effect to performance.

The performance level is also changed for following dividers:

1:2 Performance level changes from level 2 to level 1
Level 1 is the tightest possible, so expect some huge bandwidth

Dividers 3:5 & 1:2 are FSEL strapped to 800FSB.

At 500FSB the performance level of all dividers are set more loose.

1:1 Performance level changes from level 6 to level 10.Level 10 is the loosest one. Usually the chipset can handle level 5 just fine

5:6 Performance level changes from level 5 to level 8.
Setting level 4 should not be a problem.

4:5 Performance level changes from level 4 to level 7.
Setting level 4 - 3 should be fine.

Since the memory frequency would get a bit high at bigger dividers I don´t have actual values for them. But I recon the values would be following:

2:3 Level 3 -> Level 6
5:8 Level 4 -> Level 7
3:5 Level 2 -> Level 5
1:2 Level 1 -> Level 4

Does your stock running DDR2 feel warm-hot to touch?

Old news to me, but maybe very new to others.
The current being put through a module of RAM can also dictate how high you can clock your RAM or in same cases no difference at all. Depends on luck(unknown factor) and what chips are used.
D9 series chip are particularly vulnerable and is the cause of 680i burning up RAM on stock vDIMM(1.8V)
Under DFI boards the option for it is oftenly refered to "Drive Strength" with two selections 'High' and 'Low'.

So if your RAM is running above average temperature (by touch) at stock settings, see if there's an unusual option that doesn't relate to timings on RAM. If not.... change your mobo to a DFI?