We currently host our external servers at a vpn provider and have 4 hyperthreaded cores (E5-2690 3.0Ghz) and 4gigs of ram. What do you feel would be the impact of switching to a Dual Xeon 5420 (total of 8cores/8threads) with 16 gigs of ram?
While the Xeon 5420s are pretty old by todays standards, does the extra core count and huge increase in ram warrent the speed/efficiency loss from migrating away from the 4 threads on a 2690?
Definitively. The RAM alone will speed things up for you.
None of this really matters (past a certain low point like 2GB RAM, 2 cores, unless you have an enormous database), what matters to Discourse is single threaded performance.
At least that E5-2690 is from this decade, it is Sandy Bridge.
Xeon 5420 is positively ancient. It’s from 2007. It will be massively slower, even with {infinite} cores.
Besides of performance, you need to keep in mind that the E5xxx platform is really old that getting replacement parts is harder than recent devices. It may probably be fine if that E5420 server is managed by some hosting provider that has a parts pool. But if you are going to buy and run that server by yourselves it is going to be a big impact that you need to consider very carefully.