Is it a good idea to buy old / high PR domains and 301 redirect then to my website? Will it improve my rankings?
You can buy featured directories links or classified ads site links, they are always good for ranking factor
Redirects WONT improve your rankings...and it's like going in the wrong direction as far as link building goes. If you want to improve rankings, tweet, bookmark your pages, submit to good directories and niche directories (not the free spa##y ones with junk listings), create a blog...and promote: word of mouth, by images, in articles...without spa##ing of course.
No if your Redirect on your site but use same content same linking and same strategy bcz PR is depend on content, website backlink, popularity. also one thing u can 301 redirect all old website pages to new website so its better for u
THe best thing that I know of for old domains is if you can get them, use them to build another website, then once its got good rankings and the such, put it up for sale. Great way to make a few dollars on the side.