Upgrading Graphics & Sound Cards for a Hardware Nub

Hi there,
I'm a complete nub with hardware. There are several events that have made me realize I need to upgrade my sound and graphics cards:

1. According to "Can I Run It" (Can You RUN It | Can I run that game | Game system requirements), my current graphics card doesn't allow me to run the newest games like Modern Warfare 3, etc. I've had my current desktop since fall 2008.
2. Whenever I try to run a full screened video (e.g., on YouTube, in VLC), the frame rate chugs and the sound is choppy. I've gotten used to never watching anything in full screen. Some of my hardware savvy friends claim that there's interference from my sound card that screws up my graphics card.
3. It's Black Friday! Deals FTW.

Here are my specs:

• HP Compaq dc7800 Convertible Minitower
• Intel Core2 Quad CPU Q660 @ 2.40 GHz
• Nvidia GeForce 8400 GS
• "High Definition Audio Device" (Sound card. Not sure what it is. Device Manager doesn't give useful info. Do I need to open up my computer?)
• 931 GB, 233 GB hard drives
• Dual screen monitors. One is 1024 x 768 x 75 hertz, other is greater resolution.
• Windows 7 Home Premium 64-bit

So, I think it's in my best interest to upgrade my graphics and sound card. Unfortunately, I have no idea
(a) what sets of graphics and sound cards are compatible with my computer and
(b) what is the optimal purchase?

If you respond, could you please walk me through your process and recommendation?

So many factors!

The sound card is currently on-board and would probably be fine if you found the manufacturer drivers instead of the generic ones.

The graphics card can be any PCI-E up to 16x (assuming PCIE2.0 compliance, I couldn't find this out from HP) but remember your stock power supply probably wont handle a powerful GPU and may not have a 6 pin PCI-E connector to give the card supplementary power.

If you are going to buy a graphics card, I'd buy a PSU at the same time.

Possibly http://www.newegg.com/Product/Product.aspx?Item=N82E16814130625 and http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005

to get the Pair for under $200

EDIT: Just had a thought your current case may not accommodate the larger size GPU