Ensure that you have bios version 0505 or later, think 0505 is the release bios, dunno how you will check this without a post screen though.

Ensure that the CLRTC jumper is in the normal default position jumpering pins one and two

Ensure that the chassis entrusion detection jumper is in the default position, connecting "chassis signal" and "ground"

Ensure that you are using the correct PCI-E power tap from the PSU, some ati products have a 6 pin, saphire product page for the 4850 shows a 6 pin power tap; if you have a modular PSU, ensure that the PCI-E power tap is connected to a 6 pin power tap on both ends. If your board has built in video, manual shows rear connector number 8 DVI out, try connecting your monitor to this yet?

Any chance you can verify that the memory/video/cpu are working in another machine?
Use your 4850 in the blue slot with the asus vga switch card in the black slot, to enable 16x
have you tried either videocard monitor plug?
Is the monitor turned on? does it have a "source" button, tried that?

No offence on the obvious stuff, or stuff you may have already tried. These are the steps I would take. Verify that the other components are working in another machine, then check the jumpers, power taps on the motherboard.. If things test OK on another machine, it sounds like RMA time for the CPU or mobo.