600W is not enough detail. Let's get make/model or picture like this one.
http://itshootout.com/kuzals/maxtoppsu/psu2.jpg
If could be the old board doesn't support such a new video card.
http://www.tomshardware.com/forum/333475-33-radeon-5670-graphics-card-boot notes a common old issue.
Bob
First post so please be gentle.
I'm trying to upgrade the graphics on an old Intel D915GAV board from on-board to PCIe. I recently upgraded the PC from XP to 7 Home Premium and Intel does not have a Win 7 driver. The basic VGA works but leaves much to be desired.
I have an XFX Radeon 5670 HD graphics card that I just took out of my primary PC that works fine.
The problem is when I install the 5670 in the PCIe slot on the Intel board the PC won't boot up, not even to the BIOS. The fans come on and hard drive spools up but that's it.
The board has a single PCIe x 16 slot.
I've checked that I have the latest BIOS.
The BIOS should auto-detect the PCI graphics but I also went into the BIOS and set it to PCI but it made no difference.
I've removed all other PCI cards (Firewire and Linksys WiFi) but still no good.
I haven't removed the power supply (label not visible when installed) but I believe it is 600W.
Any ideas what the problem may be?
Any help would be appreciated.
Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic