EDIT: Read this article about a hotfix and otehr ideas on overclocking that card:http://forums.extremeoverclocking.com/showthread.php?t=310561
Honestly, I think those cards may not be meant to overclock. ATI tends to limit that on low end cards. I think it's a situation of its components can't handle the stress over any overclocking.
This article shows some overclocking done on them, but that doesn't mean every card will do it.http://www.legitreviews.com/article/652/12/
This person mentions overclocking theirs, but good airflow is needed.http://www.overclock.net/ati/394286-ati-3450-they-good.html
So, without more information on your system, it's hard to know what the issue is.