Computer reboots after adding second stick of ram

hello, i have a problem when i add a second stick of ram to my computer, it reboots randomly weither im in my desktop or before i even get to windows.
i bought a brand new stick of 256 pc3200 400mhz ddr ram to go along with the 512 already in. now i had this problem before with another stick. (i thought it was because it wasnt compatible because the stick were different so i went out and bought the same sticks. but when i put in the second stick computer just reboots randomly and it isnt because its a bad stick of ram because they were tested. oo and computer runs fine with just the 1 stick.(either stick i use) any suggestions would be emence thanks

 

how many dram slots does the mobo have? if more than two then play around with the different slots and see what happens.

what mobo is it?

 

i thanks for replying
it only has 2 slots

the mother board is
Socket 478 Intel® Celeron/Pentium 4 400/533/800MHz FSB, ATX, VIA PT800 + VT8237(2) DDR 266/333/400 DIMMs (2GB)
(5) PCI (1) AGP
Video: AGP4X/8X
Audio: 5.1CH
Features: ATA133, SATA RAID, USB2.0, LAN (opt)

its a chaintech soho

 

Providing your system specs could prove very helpful

You claim the stick you bought is exactly the same.
Being the same brand & even model # doesnt necessarily make it the same, based on your post, at the very least it's different size.

Look at the mem sticks & see if the same exact marking are on both, other than size.
There are many different types of chips used for memory, even of the same Brand, not all can support the same voltages & timmings.

Another way to see if your memory is different, run each stick by it's self, letting your system bios determin it's speed by SPD or auto settings, then run Everest & note the timmings & voltages.
In Everest look at: Motherboard / Chipset, to see mem timmings
Everest:
http://majorgeeks.com/EVEREST_Free_Edition_d4181.html

 

my first stick i have in my computer which i had before i baught the "new" stick is PC3200 DDR RAM 512 400MHZ the "new " stick is pc 3200 DDR RAM 256 400mhz they r both made by ULTRA i tested both sticks 1 at a tiem in both slots they work seperatly but not together is there anythign i can do?

 

Not really. You can try them together in a second PC, but incompatible RAM is incompatible RAM. They're probably different lots. Different enough not to want to work together.