How has it been determined that the slowdown is the fault of your graphics card? Have you looked at temps for your processor yet? If your graphics card is going to malfunction, it will usually do so with pretty telltale signs. You can have driver errors because the hardware is flaking out, BSODS, random reboots, freezing, multi-colored and random seeming patterns of garbage or distortion on the display, and program crashes while running applications that utilize the 3D portion of the graphics card.
From your description, you've gone from reasonable frame rates down to paltry frame rates, but nothing other than that as a symptom. If the graphics card was causing that, I would initially suspect thermal throttling. An idle temperature of 60°C doesn't sound unreasonable at all for a single slot 8800GT, even if it's quite clean. The initial run, if anybody remembers was done with an inadequate sized fan in the cooler, and was later changed to a much larger fan which was both quieter and far more effective. A quick search on Google for BFG 8800GT shows that it has the smaller fan, so expect higher idle temps and noise. What's more important to know is the temperatures you're reaching under load.
To see your GPU temperature under load, if you don't already have a method, you should be able to use a utility such as
GPU-Z which can show a graph over time of such things as temperature, and even log the numbers if you wish.
NVIDIA states on their page for the
8800GT that it's max temperature is 105°C, so you have plenty of headroom over 60 if you need, but whether it's going to run into thermal throttling at some point, I can't remember anymore. It's likely that in the 90°C and up range you could start to see throttling to try and control temperature.
It's also possibly that your CPU could be throttling due to high heat. I would check your processor temperature while you're investigating.
Do you have any programs installed that could be making use of the CUDA cores on your 8800GT? It's always possible that some software has inadvertently used some processing resources that are not being released for your game as they should. Modern web browsers would be an immediate suspicion, as most of them use some form of hardware acceleration, and yes, can easily interfere with your GPU's ability to get other tasks done, or done correctly. Make sure you're not running web browsers in the background, or other tasks that could possibly be accessing your GPU. On that note, even tuning software such as MSI Afterburner and FRAPS can cause random glitches and odd behavior from time to time. I would suggest to not even run this during your troubleshooting, as you could be causing your own issues without even knowing, outside of monitoring for temperature or GPU usage percentages.
You haven't mentioned if the 8 FPS is now a constant, or if it's after a certain amount of time of having the system power on and using 3D applications such as games. When the system is cold, such as after being off for several hours, do you regain your expected FPS in your game? It could be that your 8800GT is dumping too much heat into your computer chassis than can be adequately dealt with by the Dell's original cooling. As stated in this
Tom's article, Intel states your computer's internal temperature must be at 38°C or less, or your processor may exceed it's thermal design limits.
I am leaning away from suspecting your power supply. In my experience with 8800GT cards, when inadequate power is fed to them, they tend to produce an audible beep. If you don't believe me, try booting your computer with the 6-pin PCI-e power connector disconnected from the card.
I would also suspect inadequate power would cause driver crashes and TDRs, as the GPU would likely be spitting out bad data left and right.
Other things you might look at: RAM usage when your system is running slowly, hard drive usage and free space, if other tasks are using large amounts of CPU cycles. Antivirus software? Disk defragmenting going on? Windows updates?