|"Are you're F keys disabled?"|
Some keyboards have a button that toggles F keys on/off - if it does, it probably has an led that lights when they are enabled.
That may be enabled in Windows automatically, but not while booting, and you need to press the button.
Repeatedly press F8 very early in the boot sequence - don't hold it down.
If you have any bootable CDs or DVDs in drives, take them out - many bioses will boot the CD or DVD if you press ANY key at the appropriate time (often anytime within 5 seconds after the text prompt for that appears) while booting.
Were you fiddling with the ram as well since you had the Nvidia card in it and it was working fine?
If you were, or in any case, try this:
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
Did you change which ram you have installed? Did that ram work fine with the NVidia card?
You MUST connect an extra power connector from the PS to the mboard, other than the main one, and you MUST connect an extra power connector from the PS to each of the two cards. Did you do that?
What capacity does your power supply have?
You probably need that to be at least 550 watts minimum for two 4850's - see the specs for the cards on the card manufacturer's web site - that info is often under System Requirements, and it often also specifies a minimum current at a voltage the ps must be able to supply.
What chipset does the mboard have? If it's NOT ATI/AMD, then you can't connect the two cards together with a Crossfire cable connection - the mboard does not support Crossfire.
Once you are able to press F8 and get the boot menu where you can choose a mode, if it won't work in Safe mode, or in any case, try Enable VGA mode - if that works, un-install the NVidia video drivers in Add/Remove Programs - you may not be able to do that in Safe mode.