Well I had complete success with this last night. I now own an SI capable machine!! Thank you anatolymik & everyone else involved in this. Absolutely brilliant.
I'll go into detail, I'm not actually sure if all steps were necessary but I really wanted this to work, after getting a second GFX card and new PSU!
I did a clean install, I actually just shrunk my primiary partition, because I didn't have room to backup and do a complete reinstall. Which was really easy, just made a spare 100GB partition, through device manager/disk management.
Made sure APCI 2.0 was enabled in BIOS before a clean install of Windows 7 x64.
The way I installed both SLIStringInstall and Halmod was; I went into start menu/accessories found CMD, right clicked and ran as Administrator.
I installed SLIStringInstaller v1.1, with the above method, clicked ASUS, rebooted.
Installed Halmod v0.8, again with the same method as above, rebooted.
Installed latest Nvidia drivers from there site. The drivers had almost finished installing when I saw in the corner "You are running Multi-GPUs in Single Mode" Or something along those lines.
SUCCESS! SLI is enabled!
I didn't disable UAC, didn't run anything else just did exactly as I've outlined above. Couldn't have been easier.
Benchmarks coming after work tonight!