Website issues in IE6...what to do?!?!

Well i have finally got the bulk of that website finished and its looking fine in FF and IE7…but its all over the place (seriously…all over the place) in IE6. I realise im a total novice when it comes to this as i started the website in XHTML 1.0 Strict…but somebody in work said it would be a better idea to use Transitional? They also said i just need to change the top of each html page…

from this:

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
<html xmlns=“http://www.w3.org/1999/xhtml”>
<head>
<meta http-equiv=“Content-Type” content=“text/html; charset=utf-8” />

to this…

<!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.01 Transitional//EN” “http://www.w3.org/TR/html4/loose.dtd”>
<html>
<head>
<meta http-equiv=“Content-Type” content=“text/html; charset=utf-8”>

Is that ok to do that?

Also…is there a method of checking the end-users browser and forcing them to upgrade to IE7?

EDIT - I have also checked the browser compatability in Dreamweaver and the only errors are 3 “Z-Index Bugs” (Which i was told in the tutorial that i completed they were ok???)

I also asked the web designer at my work about forcing a user to upgrade…she said that wasnt possible…but my mate who works with me said he has seen websites that if you dont have a specific browers - it takes you to another page? Anybody seen this…cause that would be ideal.

Thanks in advance folks