Swedman, here are the crazy things I'm wondering if you can do: a comments system with moderation for editing and banning. Number of file downloads (sort by this). Search functionality. Announcements. Did I miss any? Anyhow, I'd like to see the site looking as close to the above picture as possible. Shoot me an email
Where 'x' you'd insert a catagory name like Skins, Mods or Maps.
640 is a decent size... 6 years ago. Today, almost everyone has at least a 19" monitor and most people don't get to use the real estate on their widescreen monitors. I'm sorry, but if you aren't running a decent resolution in todays day and age, you probably can't run minecraft anyway.
Wolfx seems to really want his design, so I'll throw mine up anyway. I have the psd, so size/color/anything can be changed, and I really don't expect to be used, but nobody seems to be trying for wolfx's design.
1000 px is a good width today unless you are targeting phones. Wider then 1000px is a bit hard to read depending on the content. If you want to use wider than 1000px you might as well make it use 100% of the width in all sizes.
800px is actually pretty dead and buried, very few people using minecraftforum.net have such a low resolution. 1000px is perfect, if you have 800px people like me (with a 1680 monitor) will have a ton of ugly space.