Here you can find a lot of Top Joomla Extensions... Most of them really helps you in SEO. http://extensions.joomla.org/extensions/top-rated
You can use also some from these joomla seo extensions - in this category are interested extensions - free and paid.
My best joomla extension ever is Simple related article plugin, this plugin allow you to link articles from page to page and provide a space for you to put you adsense code so that you can display ads beside your related article links. This plugin does not only link your articles from page to page it also increase the income of your joomla site and also provide a great advantage to get highly ranked by search engines , this plugin is free and not link is attached to it so it is purely free it is compactible with joomla 1..5, 1,7 and 1.6 the version for 2.5 is yet to be released, to get this extension you have to visit joomla extension directory and search for simple related article plugin, download and upload it to your joomla site after the upload you can add your adsense code by visiting the plugin backend setting and pasting you adsense code to the space provided for code input I will recommend 250 by 250 rectangular ads and to display 5 articles per page, make sure your articles keywords are related to each other because this extension make use of keywords to link article to each other
Here is my list: 1. Xmap to create a sitemap. 2. ScribeSEO to optimize the article - great tool but only for Joomla 1.5 it's a pity 3. Sh404sef - can't find anything better for url management and page redirection. 4. J4age -Monitor traffic statistics
SH404sef is the best, with lot of options for site analysis, custom error pages and ease of use. It has wide array of features dedicated to SEO.
I think Joomla 1.5 is quite good enough for you.The latest versions has much better support for titles,keywords etc