It sounds like you may have a heating issue.  Before you rule out the overheating video card, try updating the drivers.  Go to www.ati.com/support/drivers to download the catalyst 3.9 set.  See if the drivers improve performance, if not, then you have to worry about either your cpu or your video card.  I would recommend opening the case and make sure the fans are moving as they should.  Do you notice any loud noises coming from your computer? 

The symptoms your computer is suffering from is typical of an overheating problem.  Is your system still under warranty, or did you get this system from someone else?  If still under warranty, you might want to have Dell send a technician to your house and check out the problem.  Dell will supply you with new parts where needed. 

After you try the new drivers, post back and let us know if there is an improvement.

As for the dust situation - you can pretty much rule that out of the equation.  It is normal for a system to get caked up with a little dust.  The dust will not have that much of an effect on your computer from overheating.   Yes, it will start to restrict airflow, but the problems you are encountering are caused by heat fluctuations that go beyond what dust can cause.  Second, if the compound that holds your heatsink to your processor is wearing away, you should still be ok.  The compound makes sure there is a "seal" all over the processor and the heatsink to get the most heat transfer out of the heatsink.  If Dell believes this is the problem, then have them take a look at it.  Although removing the processor from the heatsink is not a difficult task, you can cause damage if you don't know what you are doing.  As for the fans.  If you believe that the fans are the culprits, then DON'T replace them on your own.  Especially since you are under warranty!  If you were to replace a fan, then you void your portion of the warranty that will cover your computer's ventilation.  Allow a Dell technician to replace the part or have them send you a new one.

What is really odd is that the sound gets noticeably louder.  The fans in your computer spin at a constant rate which will give you a steady "whir" type sound.  If there are noises that are new to your system that have started as of three weeks ago, then it sounds like your bearings are wearing out.  I would tend to rule out the processor/heatsink compound in this case.  I would rather focus in on the fans.  Call Dell and have them walk your through the troubleshooting for a noisey fan.  They will have you unplug the power supply and case fans to better locate the noise.  If all else fails, have Dell send out a technician to replace whatever parts that may be going bad.

I'll check back later with you.  It is pretty late on the east side and I'm sure you will get this message since you are on the forums.

i mention the heatsink being glued to the cpu because i really wanted to replace it. if i turn my pc off, and turn it on when i wake up, i cant hear the fans, but when im doing a lot on my pc, such as gaming, they definetly get much louder, but on the other hand, i leave my pc on 24/7 so it hasd a lot of time to idle when i sleep, yet the fans remain faily loud, but not as loud as when i game. it has always been this way, i havent noticed any changes in noises. im on the east as well and very tired. i will open the case sometime early this week and get back with my findings.