First of all a huge thanks to everyone making this possible!
In particular for everyone using ATI HD4850 graphics herewith my steps leading to success:
1. Bought compatible HW (GA-H55M-UD2H + Core i3) and decided to give it a go with my HD4850 graphics and IDE DVD-drive
2. Boot with iBoot-CD but trying to run 10.6.3 retail (on USB-drive) did not work.
3. Removed my HD4850 card (don't forget to re-activate board graphics in BIOS first!) and voila: it goes!
4. Installed 10.6.3 (for time being to another USB-drive), reboot with iBoot
5. Installed 10.6.5 combo update, then straight away ran multibeast with following options:
a. UserDSDT with appropriate DSDT on desktop
b. Advanced options: ATI48xx device injector + Netkas 48xx patch for 10.6.5
c. I skipped anything related to onboard audio since this is turned off in the BIOS (using Delta card)
6. Re-ran Multibeast only with System Utils (rebuild caches/repair permissions)
7. Turned off PC, re-installed HD4850 (adjust BIOS to initialise PEG-graphics first)
8. Boot straight from 10.6.5 and OMG this actually works!!
9. Installed lnx2mac's Realtek 8xxx driver and also some Envy24 driver for the Delta 2496
10. The IDE-drive still works fine OOB (no need to install Macman's JMicron 36x kexts in Multibeast?)
11. sleep, speedstep not tested yet... still need to move installed OS to internal SATA drive
Conclusion: happy I went with H55 chipset: without the board graphics I would not have been able to get the HD4850 going this easily!!!