I know that keywords in domain are good for some search engines. But what about keywords in subdomain? I want to make a site with different sub-topic (same main topic), one sub-topic for one site. I want "mysitename.com" for my main site and I would like to seperate these sites with keyworded subdomain. My doubt is "Do keywords in subdomain weigh the same as the keyword in main domain?", like "subtopic.mysitename.com" ("subtopic" is a keyword), is the keyword really help? Or it is better for me to buy several keyworded TLD for different sub-topic and link them to the main one?