what are the diffences between 2 Duo T5550 n 2 Duo T7250

Actually, you have already list the some of the differences,

1. Clock speed
The clock speed of T5550 is 1.83GHz while T7250 is 2.0GHz
However, This small difference can just affect about 5-10% in performence, you can feel a little bit different in performence due to this. Since I do not have the price difference, I cannot judge if it is worth.

2. Front side bus
The front side bus speed for T5550 is 667MHz while T7250 have 800MHz, the front side bus is the link between the CPU, motherboard chipset and RAM, however, this different is not really noticeable for normal purpose, as you are using the RAM with 667MHz, which do not cause the FSB to be the bottleneck, the performence difference should be less than 5%.

3. Intel VT(Virtualization Technology)
This one, it is not a matter for normal users, because you are not using a windows with another windows on top(Will you?). For example, you are using windows vista, but you install a virtual windows XP on top of this, then this technology can affect you a little bit. However, it can affect only 5-10% in performence even in this case. For normal prupose, it will not have a noticable different.

For the 3GB Shared Dual Channel DDR2 at 667MHz, and 4GB Shared Dual Channel DDR2 at 667 MHz. It depends on how you use the computer, for example, do you usually run out of memory when you play games etc. (You can check this in Process manager)? My experience is that if you use it for normal studying purpose, 2GB must be enough for both vista and XP. For gaming, 3GB is also enough for XP and some of the games in vista. But if you say games with high requirement, better to be 4GB in vista (I do not know which type of game you usually play, unless you told me).

depends on what you do with your computer