Need help with redirection issue

Hi ,

i need your help for my issue please:

- i have a website which is PR5
-I bought a website (busy one PR4)

i don like the name of the site i bought and i have a better one to use .

I will forward both sites to the new domain name , what will happen in this case ?

-To the PR ?
-To the links in search engine?

The final design will be similar to the PR4 site with some changes .

What will happen to the engines backlinks to the sites ?

both sites will still be registered..

is is better if i host these sites and forward them to the new domain ?

or is it better to forward them on the registrar basis ?

any idea how to manage without dropping on the SE ranks ?

thanks