I have the 8400 also.
here's the RAM i got with it.
1GB DDR2 SDRAM @400 MHz
I ended up adding 2 more GIGs to make a total of 3.
But yes, use the Crucial memory checker, thats what I did and it works great.
I just went to FRY's electronics and got my new ram, but going threw Crucial would be better i think.