The idea of getting 2 servers is not bad, but the problem comes to keep them both UP at the same time, thats why people usually go for 2 x database and 1 web server. As if you invest in one better hardware with redundancy options intact, it would be much better solution.
Another possibility would be going for a cloud platform, and you can upgrade/allocate resources as when needed.
so I assume its a dedicated server bought at a hosting provider? how much is it you are paying? As far as MariaDB and Litespeed are concerned, both are drop in replacements for MySQL and Apache respectively, means the SQL queries that you use in MySQL can be used in MariaDB as it is and same goes for Litespeed, all apache's commands work in Litespeed.
There is another webhost (www.digitalocean.com), they do smaller scale cloud nodes, what you can as well do is, get a bigger node for web-server and moodle data, and 2 smaller nodes for MySQL (say 1 core + 1GB RAM), there could be plenty other options available with plenty other hosting providers, all that depends on your situation if you are doing co-location (means your hardware and hosting provider's network/UPS etc) as I type, I am not fully aware of your exact path in terms of hosting provider and the money you are spending, therefore I still need some more info in terms of what I can exactly suggest.
This moodle.org (as of 2009) webserver was based on 2 x Quad Xeons (4 cores), 2 x 146GB SAS in RAID-1 with Hardware RAID Controller and 8 GB RAM & Database server was based on 1 x Quad Xeon (4 cores), 3 x SAS Drives in RAID-5, 8 GB RAM.
You have 5 times better CPU and option of SSD drives, I believe with you even 64GB RAM will be enough.