Just for testing change the bios setting so the DVD drive is the only boot device, then set it so the DVD drive is the first boot device and try again.
Some boards are a pain to get to boot off of CD-DVD drive.
My second thought is that the boot-section of your DVD did not copy correctly. Most home made DVDs do not boot correctly. If you copied the original using a free CD copier or just copy and pasting from windows the boot section probably did not get copied.
Nero does a good job at making bootable disk. Or you can use Windows to copy the disk as a iso or img and then burn the img.
My gut says its the DVD, do you have another bootable CD-DVD like a windows xp install or even a memtest dvd. If those boot that would point to the win7 dvd being bad. If those don't boot then the issue is with the bios settings or the dvd drive.
Just wanted to let you know you were far from the only person with this problem. Just bought my parts to assemble one this weekend, nearly identical setup:
-- Phenom II X4 970 CPU
-- Gigabyte 990FXA-UD3 (revision 1.0) board
-- Corsair XMS3 8GB (2x4GB) DDR3-1600 RAM
-- WD Caviar Blue 1TB drive (no RAID)
-- Thermaltake TR2 750W PSU
-- XFX Radeon HD 6450 1GB video card
-- Windows 7 Professional 64-bit OEM
Had the same problem -- the parts were all connected just fine, I could hear the hard drive spin up, & the DVD spin up in the drive...but after the boot/POST it jus said "Loading Operating System". Testing the various hardware was really difficult, since the PC I was replacing didn't have a SATA controller, both new DVD & hard drive were SATA, old DVD & hard drives were IDE, & the new board didn't have an IDE controller. I got as far as verifying the DVD disc was at least good -- my old PC told me "Your processor isn't compatible with this type of operating system" -- but was just about ready to take everything back.
But then I reconnected the new PC, did a quick search, & found that it was the fault of the BIOS. Gigabyte's forums had a thread with a humongous amount of posts, basically saying that the most recent BIOS (version F5B) was the cause of the problem. That BIOS was the one just released this month to allow the boards to take the new FX processors...& "to be helpful" Micro Center had just flashed the update onto all of the boards.
Flashed it back to version F4, & it immediately recognized the DVD disc & started installing Windows 7.
I have a very similar issue! I am a 1st-time computer builder (I recieved all my parts yesterday (10/27/11) which consisted of an AMD FX-8120 chip, GA 990FXA-UD3 mobo, Corsair Vengeance DDR3 4gb sticks of ram (x2 Model: CMZ8GX3M2A1600C9), Saphire Radeon HD 6770 PCI express card (Model: 100338L HD6770) Rosewil green series Power Supply (Model: RG 630-S12 ), & a Seagate 1TB HHD (Model: SATA6.0 32M ST31000524AS). After probably 4 hours of assembling (yes I know that took awhile, first time I even touched a mobo!) I finally had everything where it was suppose to go and I boot it up, all seems well so far!, A few seconds after the Mobo loading screen I come a screen which says "Loading Operating System..." sometimes it will reset to the mobo loading screen and go on a rebooting loop, other times it will stay stuck on the "Loading Operating System..." screen. I am trying to boot my os from a USB Flash drive (2.0, not the new 3.0ss USBs) Ubuntu (I have no optical drive atm, but will be buying one possibly today after work). Mind that I have messed a bit around with the BIOS settings (little troubleshooters that I have read in other forums which aren't working) and still nothing seems to give. I am an intern in a company in the IT department (as that is my major) so I asked someone here and they suggested taking apart some pieces such as the graphics card first and see if it boots from there, also my RAM sticks but I have not had the chance to attempt those troubleshooting steps yet. Once I get home it will be Round 2 of trying to get this computer up and running perfectly! I just want to ask what are your guys' suggestions on what I should do to troubleshoot. I've read around that flashing your BIOS to a new one is probably a good bet. My question, how do I flash my BIOS with a new update? how can I check my current BIOS version? What suggestions do you guys have? ALL INPUT IS APPRECIATED!=D