You won't be able to put an AMD processor in an Intel socket. Very doubtful whether you will be able to get a different motherboard to fit Shuttle case so you will be limited to whichever Intel processor is the maximum that the board will take.
Why not invest in a budget base unit which will be a complete upgrade and better than what you have? Depending on what you use your pc for, In my own personal opinion i wouldn't spend money on upgrading it.
For a start, you can't even upgrade to 2gb Ram and I guess you might even have built in onboard graphics and sound. Its like swapping a 1.6 engine to a 3 litre one but without changing for example the brakes, exhaust or gearbox.
I hope that you are not put off by the one unhelpful reply which you received. Most of the forum members are only too happy to help if they can. We can all learn from each other no matter what level of expertise we have.
AS above you are limited to Intel CPU's But you could fit a Upgrade Bundle Maplins do these in Intel and AMD but don't forget that the OS will not be no good if its tied to the Computer Plus you may also need a new Graphics with a Mobo upgrade
Please note the machine is a Shuttle. This is a small form factor PC and motherboards for them are as rare as hen's teeth and expensive as well. I have got my son's one sitting beside me for which I was looking for a motherboard and he has decided that he can do much better if he goes back to a standard case. One of the limiting things with Shuttles is that there is not much room inside for expansion and the lack of room makes them run rather hot. Upgrading when your stuck with a 220 watt power supply is also problematic. Connecting a USB powered external drive started my son's problems as that power supply is almost at it's limit anyway.