Hi,
Are you seeing anything wrong in the ERROR logs of your server?
And do you have http://localhost/ in your local computer?
You can install one through
- EasyPHP (for Windows)
- XAMPP (for Mac OS x)
Those are freeware software and they both comes with PHP5.3.xx
Just give it a shot and try to install the DarkMart in your local machine where PHP5.3 is being used, you would notice the difference almost instantly.
The joomla Framework that has been used might be reason of that slowness with PHP5.2, this doesn't change the fact that joomla and virtuemart are working okay with PHP5.2, they can but it is the "minimum" requirement.
But if you look at the "requirement" page of Joomla!:
www.joomla.org/technical-requirements.html
You would see, they recommend PHP5.3
Regards.