My monitor doesnt detect my pc

Hi,

here is my story,

i used to have a old pc

amd athlon 64 x2 4200+
ati 3450
2 gig ram
msi k8n neo4 platinum
300watt psu

as you can already see its old, however i made the mistake of buying a new ati 5770 gfx card for this pc. Everything went well i uninstalled my old gpu, removed it, put the new one in. But when i turn my pc on the screen works for 3min and becomes jaggid and unresponsive. I tryed to restart but to no avail.
I figured it must be my psu, so what did i do? i bought a new psu, a Corsair 600W CX PSU - 2x PCI-E 6x SATA.
So i took out my old moldy psu and put my new glorious corsair in, now here is the problem my pc starts fine but my monitor, which is connected via a dvi converter to my new gpu, doesnt detect my computer.

However before i had installed my new psu it detected it find, it just could not run it.

So now here i am asking my self WTF? is going on and what should i do.

Help would much appriciated, thank you.