asus m4n78pro
amd phenom x4 9850
oczsli4096 ddr2pc6400 x2
coolermaster gemini 11coolermsidvdrw w/lightscribe
wd caviar blue500gb7200rpm sata
fdd cd R
evga 9500gt 1gb ddr2
win7 64 bit

is it best to install the operating system before installing gpu and other add on cards?
and should I fire up the pc for a first time before installing OS

Follow the guides on this website: http://www.buildacomputerguide.com/I'm assuming this is a brand new PC, not yet assembled?
If not then the below would still apply, just not all of it if that makes sense.
Others MAY have different methods, but my personal way of doing it would be to install all the hardware first and of course double or triple check all the connections.
Fire it up, with your KEYBOARD, mouse and monitor connected, enter the BIOS, make sure all the hardware is detected properly and change any settings you need to change.
Most importantly, set your DVD drive as the first boot device so you can boot from your Windows disk.
Insert the disk, save the BIOS settings and reboot, boot from the disk, and you're all set.
Once Windows is installed, connect anything else such as printers, webcams etc and install the appropriate drivers.
Windows 7 includes default drivers for most hardware out there which will be enough to get you started, if UPDATED drivers are available use those though as they will give better performance and/or more features.
Hope this helps

Edit: I need to type a little FASTER, it seems I was beaten to the punch (post?).Thanks KPAC for the Link thats the first build site that said I wont need to format my hdd before installing theOS. other than that. as soon as my OS gets here I'll be ready to do a first start.