The Intel® Core™ 2 Duo E6400 has an FSB if 1066 and support dual channel memory so you are going to want to use DDR 2 with a speed of at least 533 (pc4200). You can use 667 or 800 based on what the board will support but you do want to put in in matching pairs. As far as it bottlenecking you would have to know the other components (i.e. video card). I would try to have at least 2GB of memory for a current system and moving up to 4GB if the you have a 64bit OS.
Go with 2x2GB DD2-1066 (PC2 8500), set to 800mhz and then overclock the cpu, keeping the memory at 1:1, cpu voltage no higher that 1.4v, should hit about 3.6ghz with good cooling. A lot of good guides over at extreme overclocking.