Asus P5VD2-MX mobo won't boot after installing HD7750

Have installed brand new HD7750 into the below and now pc doesn't seem to be booting.
Don't think my mobo has a speaker (has never beeped),
Graphics card fan starts up and stays on
Monitor won't turn on at all - using DVI output with adaptor to VGA (I don't have a DVI cable for my monitor yet) have also tried HDMI output to TV but nothing

Onboard graphics work fine as does the NVIDIA GeForce 7300 LE

I chose the 7750 after asking advice from forums so don't think it should be either of the following but....

1. I can't seem to find anything that says if the 7750 is definitely compatible with PCIEX 1.0 16?

2. Since installing I have found out 4GB ram is recommended (was told anything over 2GB is wasted on my Pentium D 3.2Ghz processor so no benefit to upgrading ram), would this affect the ability to display even while booting?


I have installed the latest BIOS for the motherboard
I have tried disabing the on board graphics in the BIOS (by selecting "Disabled" from the memory allocation options, is that all I need to be doing?)

Thanks in advance....


Power Supply
Antec HCG 520 (520W)

Operating System
MS Windows XP Home 32-bit SP3
CPU
Intel Pentium D 935 3.2Ghz
Presler 65nm Technology
RAM
2.0GB Single-Channel DDR2 @ 199MHz (3-3-3-9)
Motherboard
ASUSTeK Computer INC. P5VD2-MX (Socket 775) 40 °C
BIOS
Brand Phoenix Technologies, LTD
Version ASUS P5VD2-MX ACPI BIOS Revision 1017
Date 04/30/2007
Graphics
VA2448 SERIES (1920x1080@60Hz)
128MB GeForce 7300 LE (Dell) 72 °C
Monitor
Name VA2448 SERIES on NVIDIA GeForce 7300 LE
Current Resolution 1920x1080 pixels
Work Resolution 1920x1046 pixels
State enabled, primary, output devices support
Monitor Width 1920
Monitor Height 1080
Monitor BPP 32 bits per pixel
Monitor Frequency 60 Hz
Device \\.\DISPLAY1\Monitor0