Change1: Change your table row definition from
$body = $body -replace '\n(\S+)\s+(\S+)\s+(\S+)', '<tr><td style = "border: 1px solid black; padding: 5px;">$1</td><td style = "border: 1px solid black; padding: 5px;">$2</td><td style = "border: 1px solid black; padding: 5px;">$3</td></tr>'
to
$body = $body -replace '\n(\S+)\s+(\S+)\s+(\S+)', '<tr><td style = "border: 1px solid black; padding: 5px;">$1</td><td style = "border: 1px solid black; padding: 5px;text-align:center;">$2</td><td style = "border: 1px solid black; padding: 5px;text-align:center;">$3</td></tr>'
Explanation: Added text-align:center
to the style definition of the table cells.
Change2: Just add the text at the end of the mail:
$body = '<body><table style = "border: 1px solid black; border-collapse: collapse;">' + $body + '</table><br/>All memory values are in GB.<br/><br/>Note: This is an automatically generated email, please don't reply to it.<br/></body>'
Change3: According to http://msdn.microsoft.com/en-us/library/vstudio/h1s04he7(v=vs.100).aspx you are correct: add multiple email adresses separated by ,