Here is a link to the technical specifications of your machine.
Dell 2400 specs
Notice that it has integrated graphics/video which uses a maximum of 64MB of system RAM. This is causing the slowdown in screen re-drawing. In my opinion.
The machine only has PCI slots, so if you buy a new graphic card make sure that it is PCI and not PCI-E as it will not fit in the machine.
The answer to your problem is either:
Buy a PCI graphic card with as much memory as possible. They will be hard to find. I have just checked the shop where I purchased the parts for my machine and they had only 1 PCI graphic card out of 90 products. The rest were PCI-E.
Put the cost of a new graphic card towards the cost of a new computer; or
Use another version of Ubuntu that does not run Unity.
Sorry, if this is not to your liking. It is the answer.