I am nonresident alien. I got some tax returns from IRS. $ 4000.00 for 2008
I opened account in Bank of America and deposit the money.

I've received today a letter from Bank of America that IRS requires all banks to maintain
Reports of all non resident alien. They also send a W-8BEN form to fill it .
The form is titled as certificate of foreign status of beneficial owner for US tax Withholding.
They ask me to fill out this form and sign it and return it back.
Now what the HELL is this form ? Why do I have to fill it and what happen if I ignore it ?
Also, that brings up another question : Is it preferable to transfer my money outside ( to my home country for example ) or keep it here ? I mean can the IRS find out my savings ? Does that affect my returns for the next year if they find out $ 4000.00 in my account ?

Please, I would really appreciate if someone could help me and answer these inquires for me
Many thanks..