Sure,
Put your documents in the main web directory:
/Library/WebServer/Documents
This way you'll get rid of the default pages (if you move or replace them, that is).
If you'd like it to forward to your /Users/username/Sites directory, create a redirect document to the address
www.yourdomain.com/~yourusername/ thus:
<BLOCKQUOTE><font size="1"face="Geneva, Verdana, Arial">code:</font><HR><pre><font size=1 face=courier><META HTTP-EQUIV=<font color = red>"refresh"</font> CONTENT=<font color = red>"<font color = blue>0</font>; url=www.yourdomain.com/~yourusername/"</font>></font>[/code]
Just that, no <html>, <body> etc.
If you're going to change Apache's main web folder to your own, you'll have to change the httpd.conf file. I'm not competent enough to guide you through that ordeal, though...