First thing first!
Anytime you are messing with RAM (memory) there are pitfalls of incompatibility. ....

Ram is always determined by the motherboard as to limits and speeds.
Ram is backward compatable meaning that higher speed ram that is paired with lower speed ram will always limit itself to the lowest speed installed.
Ram should never be mixed...use the same speed and brand if possible and one that is known to work in the motherboard.
Try not to buy the cheapest or low value sticks as these are the ones that will cause more issues in compatability.

You have done a lot more that what others try. You researched your stuff then you ask for clarification. Great job!

Go with at least 1 Gig of pc 2700 (DDR 333) either as 1 single stick or 2 matching sticks of 512mbs...depending on the cost and availability opt for 2 gigs as a matching pair of 1gig each.

Make sure that you have the right board as there are different Titans P4 motherboards. and since you have the manual make sure all the jumpers are properly set especially for the speed 266 or 333.

That computer should be more than enough for anyone as the graphics can still be updated to to a faster card without any problems.