[Solved] CPU upgrade = PC wont work

Hope someone can assist here. I have a 3yr old Mesh computer no idea what other the components are except that the motherboard is an ASUS P5NSLI - I wanted in get more from the system so I checked the ASUS site for a higher spec cpu - ASUS says INTEL 7300 CPU will work, its a socket 775. ASUS site says it works with all BIOS versions so I installed the the Intel 7300 chip, turned on but got no video signal at all.
The system fans all come on and it appears to be working but how can you tell? I tried another monitor and same result nothing.

Replaced the old CPU I think its running at 2.13Ghtz and everything came back on as usual after the Bios warning me that I had installed a new CPU and should enter the data in the CMOS. I wasn't sure what to do then so I set everything to default and saved and exit and Windows booted up okay. Except I now have this 7300 which apparently wont work.
Any idea what I have done wrong?

Yup thats the one.

It was brought as a complete system not built by me - Mesh Elite .
CPU I think its an Intel E6400 core duo running at around 2.13ghtz,
4GB DDR memory installed.
O/S Windows Vista 32bit
Nvidia GEforce 7600 GS
its a socket 775 - most of the on board chips are Nvidia I think.
I'm not too hot on the techie side

HEC 550TD-PTE

okay will give this a go
thanks
ed