Video card killed my computer
As things were working fine until the problem in-game I doubt drivers (or Windows 7) are your problem.
It is most likely that the video card has developed a fault (i.e. it has overheated somewhere by enough to burn out a component or few). If this is the case then hopefully it won't have affected anything else. To test the theory put in another card if you have a spare near by or can beg/borrow/steal one, preferably of similar brand and technology (i.e. if it is an ATI PCI-E card replace it with an ATI PCI-E card if you can).
Dieing this way may indicate that you have a cooling problem in your case (bad air circulation), or it could just mean that the card was a dud with a heat related fault waiting to be pushed over the edge by heavy activity and a little bad luck.
Windows 7 requires a decent video card to work properly with all the fancy Aero effects, and the default driver might enable DirectX effects instead of being a simple fallback driver.
Check the card for any visible damage; something may have been shorted or you may have blown a capacitor. If you can find it, contact the card's manufacturer immediately. The HD4830 is relatively new, and you may be able to get an on-warranty replacement.
If card has no visible damage, you may have corrupted the onboard BIOS of the card. Check to see if ATI offers a BIOS that you can use to flash the card (this might not be helpful if you can't get Windows installed).
Since your motherboard's BIOS got turned into gobbledygook, you may have corrupted that somehow. Remove the CMOS battery, unplug from the wall, press the power button 2 or 3 times, and let the computer sit for 10 mintues or so. This will reset the BIOS back to factory settings and may solve the issue.
If neither of these work, you may have borked the motherboard and the videocard. You'll have to get replacements.
If you have a spare video card (simple PCI/PCI-e will suffice), I'd recommend swapping the old card out first and trying a different card and seeing if the machine can boot and most importantly you can see the BIOS menus and such.
If that doesn't work, try resetting your CMOS values. In your Gigabyte Manual, there's a layout diagram for resetting/clearing your CMOS. Follow the directions in your motherboard manual on clearing your CMOS.
All Gigabyte boards (for quite some time) ship with dual BIOSes. If one of them is corrupt, I think there's a way to restore to factory default settings. Start with the manual.
Whenever you're troubleshooting, don't assume anything. Eliminate good parts from bad parts. Maybe one of your RAM DIMMs is fried. Try removing a few DIMMs to the bare minimum needed. Or try running the Ubuntu memory tests from a CD/DVD. Maybe the hard drive is fed up with Windows 7 and doesn't want to reinstall it for you. Try using another hard drive. Anything you can use to get the system running again. Eliminate every possible part if it comes to it.
If I had to guess what's going on, I'd say your video card is unofficially supported and your BIOS/CMOS might be a little pissed off from the whole joy of Windows 7, ATI's 'rock solid' stable drivers and an overclocked CPU. Overclocking + unofficial OS + Microsoft + ATI Drivers + BIOS is unhappy = an unhappy computer. If you can eliminate the video issue (with another card), try reinstalling again. If it fails, start investigating the disk with using another one.
I may not be able to properly diagnose your computer problem, but I can give you a few tips from preventing and possibly fixing this issue from reoccurring:
Stop overclocking; period. I've never understood the fascination with it and the "surprise" people get when a system starts to behave oddly. Why risk damaging hardware for a couple hundred extra MHz? You want to eek out 500MHz? Pay for the difference and be happy that you're not risking your hardware.
Stop using Windows 7 RC. Try something that has stable driver support .. like Windows XP. Hell Windows 2000. Linux. Anything. I don't care if Windows 7 was working beforehand. It's not working now. ATI seems to always have buggy drivers, why would Windows 7 be any exception to that history?
Don't use beta products as your primary computer or workstation/whatever (this is paritally related to #2). People can't be too much of a help on a product that hasn't even been officially released so very few people have any exposure to your problem. If you really want to play with Windows 7 RC do it virtually using VirtualBox from Sun (for free). Wait a while and let everyone else deal with the driver pains and the heart aches from it first. Once a service pack hits the streets after 6 months and people are finding some sense of stability, then consider switching.