One problem that OGA has is that the "textures" section is just a "search" form with specific parameters. However there is no <h1> or <h2> which says "Textures" or "Open Source Textures" or "Free, legal textures". The <head><title> only says "Search Art | OpenGameArt.org". This way, people using a search engine to find "open source textures" will not see OGA as one of the first results, even though it might fit their needs very well.
I believe it would be a similar effort to make OGA have appropriate tags that give it multiple faces, which would make it better-indexed for search engines or to create a new website for textures only. But creating a new website would mean double the maintenance effort. Improving OGA definitely would be much more valuable to the community.
OGA is drupal-based. If you can work with that platform and want to help, get started by getting to know the community via forums and/or IRC.
Instead of sleeping tonight I created
qubodup.github.com/ost, which is obviously heavily inspired from cgtextures (ironically, they will soon be updating their web design in the near future). All image links lead to searches on either
http://opengameart.org or
http://Burningwell.org . Who knows, perhaps it will direct search engine queries saying "open source textures" into the right direction one day