I was thinking that one the one hand, some people might prefer to just post a link to their own site with a description of their system. Others might prefer to post a description directly in the forum.
If a poster does choose to post an offsite link, they should include at least a few sentences describing the project.
Whether posting in the forum or elsewhere, I think a "good" description would include the following:
* Full list of components, including:
- Motherboard
- CPU
- Heatsink / Fan
- Memory
- Case
- PSU
- Case Fans
- Hard Drive
- Optical Drives
- Video Card (including HSF, if applicable)
- Any other Expansion Cards
- Any custom modifications or special hardware used in any of the above
* Impressions or measurements of overall system noise
* Measurements of CPU and Case temp from the demo version of SiSoft Sandra, or cpuburn, or other freely available program
* Any special BIOS settings or software used
* What alternative hardware was considered
* General sense of satisfaction with finished system, from performance, noise, and "ease of use/manufacture" standpoints
* Advice to someone thinking of building a similar system
* Pictures!

(note that you have to host pictures offsite somewhere)
Things that shouldn't be posted:
* Incomplete systems (ie, if you're waiting to get a Barracuda IV and a Zalman heatpipe for your GeForce, wait until you get it before you post)
* Systems that aren't functional/don't work right (ie, your crazy duct work is ultra quiet, but the CPU temps get so high you can't load your OS)
* If you're a vendor, information/links to systems you sell (if an end user has bought a pre-built system through normal retail channels, maybe they can post, but no commericals!)
* Systems that have nothing to do with quiet / silent computing
Things that definitely should be posted:
* Systems using a novel, custom-made quieting solution
* Systems designed for special purposes that were difficult to perfect (home theater PC, quiet home server, silent office/web browsing system)
* Systems that are cheap, quiet, and powerful
I suppose these guidelines are rather long, but I guess it's better to have them and let people choose to ignore them than not have them and provide no suggestions to well-meaning posters.
As far as allowing replies, I can't see how it would hurt, as even if the replies get off topic, they don't distract from the prominence of the main gallery post. A user that just wants to see what's in the gallery could just use the forum index to navigate from topic to topic ignoring all the replies. Even if there was a runaway thread with a big flamewar or something, the topic view pagination would prevent the replies from being distracting.
Hopefully, us users here at SPCR will be mindful enough not to post junk in that forum, and involved moderation won't be necessary. Maybe there should be an explicit policy that if you post an off topic post on the main gallery page, the moderators are free to either delete your topic or move it to general discussion, as they prefer.