Vista won't boot after installing extra ram

Hi everyone,
I have a problem that i have encountered with a dual boot
machine.First i will give the details of my computer:
ECS NF650iSLIT-A motherboard
Intel Core 2 CPU E6400@2.13ghz
2gb Ram (2xGeil Ultra 1gb DDR2 PC6400)
2xNvidia GeForce 8500GT graphics cards both with 1gb on board)
700w jeantech modular PSU
Sound Blaster X-Fi Xtreme Audio Sound card
Windows XP Home on 120gb HDD PATA
Windows Vista Home Premium on 320gb HDD SATA
This all works fine including in SLi
mode.All the latest drivers are installed and both O/S's are up to date.My
problem started when i purchased another pair of Geil Ultras (same spec as
above) to make my total ram 4gb.I installed the ramas per instructions,then
powered up the computer.Vista is my default O/S and it refused to boot up.Got
past the P.O.S.T screens then got a black screen with no computer activity
whatsoever.Powered off with power button turned off at mains waited a moment
or two then powered up again,At the choose O/S screen chose XP,got as far as
the windows XP logo screen and it just locked up on that screen.Turned off
again,removed the new ram and tested on my sons machine to find they are
working perfectley.Can anybody help,as i thought that XP could support up to
4gig and Vista even more.
Am fast losing what hair i've got left
Cheers Phil