"Graphics, is there an upper limit on how much you can upgrade a video card?"
Yes, and no gammon_ben.
Your computer uses an AGP graphics card.
Uses AGP 8X graphics cards. (1.5 volt)
Goes in the brown AGP slot on the motherboard above the three white PCI slots.
The best AGP graphics cards are the AGP 8X ones.
Sky's the limit, just depends on your pocketbook.
Yes, so far.
When using Integrated Graphics, the graphics chipset on the motherboard borrows some of the System Resources.
The Processor and Ram Memory.
The Processor and Ram Memory are trying to take care of the Operating System, and the program being ran.
(Windows XP is one example of an O/S. Operating System)
Let's use an intensive graphics program for an example, a Game, and Windows XP.
The Processor and Ram Memory is supporting Windows.
They're also supporting the characters of the game, the shading of the game, and the background of the game, just to name a few characteristics.
Due to the resources needed from the system resources, (Processor and Ram Memory), from the game, the game will lag, or freeze up.
Using a graphics card frees up the system resources.
A graphics card has it's own Processor (GPU), and Ram Memory. (Graphics memory)
(GPU = Graphics Processing Unit)
However, the Processor still has to utilize the information from the graphics card. Even though it, and the Ram Memory have been freed up, there is a limit on how much information a Processor can handle at one time.
Your Processor may be an Intel Celeron, or Pentium 4.
These are single core processors, and have their limitations.
HOWEVER one more time, you are just speaking of operating a 40 inch widescreen (?) monitor with 1080p.
A really decent AGP graphics card can handle that with no sweat, and watching videos.
Some of the newer intensive games may be a problem.
Here is an example of a decent AGP 8X graphics card, that is Low Profile. (Low in height)http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=5699698&CatId=2234