Best practices for withstanding launch day traffic burst

Measure first, and then optimize. Have you done any load testing? Where are the bottlenecks?

Once you know your bottlenecks then you can intelligently decide if you need additional database boxes or web boxes. Right now you'd just be guessing.

Also, how does your load testing results compare against your expected traffic? Can you handle two times the expected traffic? Five times? How easy/fast can you acquire and release extra hardware? I'm sure the business requirement is to not fail during launch, so make sure you have lots of capacity available. You can always release it afterwards when the load has stabilized and you know what you need.

I would at least factor out all static content. Set up another vhost somewhere else and load all the graphics, CSS, and JavaScript onto it. You can buy some extra cycles, offloading the serving of that type of content. If you're really concerned, you can signup and use a content distribution service. There are lots now similar to Akamai and quite cheap.

Another idea might be to utilize Apache mod_proxy to keep the generated page output for a specific amount of time. APC would also be quite usable... You could employ output buffering capture + the last modified time of related data on the page, and use the APC cached version. If the page isn't valid any more, you regenerate and store in APC again.

Good luck. It'll be a learning experience!

Have a beta period where you allow in as many users as you can handle, measure your site's performance, and work out bugs before you go live.

You can either control the number of users explicitly in a private beta, or a Google-style semi-public beta where each user has a number of referrals that they can offer to their friends.