The dimension 4550 motherboard supports up to 4x AGP according to spec. However, you can use an 8x AGP card as the specification is backwards compatible. It won't take advantage of 8x throughput but it should work just fine emulating 4x.
No (EIDE only)
Can't say for certain, but I am pretty sure the 4400 is newer.
It is an ATX motherboard. But even though it is an ATX size it is a company specific motherboard, and is not compatible with standard ATX cases.
T5600 Motherboard
you can buy the Dimension 4400 on amazon it is the cheapest place to buy it. it will be delivered with in 2 days. It is a very nice device because it is so fast!
2 DDR slots with a maximum capacity of 512mb in each.
One can purchase memory sticks for their Dell Dimension 4400 at Crucial, Amazon, eBay and many more.One can find the right memory card by visiting Crucial. The website has a feature which helps finding the right memory card for the desired device.
Short of buying a new motherboard it is not possible to fit a PCI-Express graphics card in a Dell Dimension 1100.
If you mean your file names are green, which is what I'm assuming, it means your files are encrypted.
Any USB printer will be compatible with your dimension 2400. Additionally the 2400 also has a parallel port, so any printer which uses a parallel port should be compatible as well.
If one wants to purchase Dell Dimension 2400 RAM that is compatble with modern computers. One must go to the site crucial, or google it. On the site one will be able to find the Dell Dimension 2400 RAM and purchase it.
It is often hard to fit an after market motherboard into dell computers, you would have to check the dell website.