Sunjyot, welcome to the Seven Forums.
To answer your questions,
1. None, see below.
The reason for saying none is that sadly any AGP card you buy will be worth more than your computer. It is your money but buying something that is obsolete (AGP has gone the way of the dodo) just for Aero doesn't make sense.
Also the motherboard has a totally unsupported chipset on it (Intel 865, of which Intel stopped supporting a long time ago) which can make getting an AGP card to install properly (if at all) a challenge.
You may not be able to afford a new computer now, but it would be wiser to save up until you can instead of essentially throwing the money away.