I'm running PostNuke 0.5.1.2 (which is based on PHP-Nuke 5.0) on e-Smith 4.1.2 without a problem using the PHP 4.0.3pl1 release. My_eGallery also works fine with that. The only "problem" you will be facing is that the GD library is not compiled into the PHP e-Smith uses, hence no thumbnails will be generated (unless you opt for the browser thumbnail generation in My_eGallery 2.4.5 release which is painfully slow!) so you have to generate your own thumbnails and upload them into the "thumb" directory within your gallery subdirectories.
I prefer PostNuke for various reasons, one of which is the open development plan and availability of a CVS where you can always download the latest version, you can check out PostNuke at
http://www.postnuke.com.
PostNuke uses .css style sheet and is completely HTML 4.0.1 complient. That said, it appears that FB just released 5.0.1 which does that too!
All the plug-ins and language files are compatible with PostNuke.
If you need help with installing and configuring PostNuke on the e-Smith server, let me know... I'm running 5 sites on e-Smith and have not had any major problems.
You can see my PostNuke site at
http://www.mahmood.tv with the My_eGallery 2.4.5 installed and functioning.