I'm running the same HP MicroServer N54L (Newegg had a Black Friday special on it). I went with 16GB of RAM despite the "official" support for 8GB - several 16GB kits have been demonstrated as reliable. I'm successfully using a Crucial 16GB Kit (2x8GB) DDR3 1333 CL9 unbuffered kit - P/N CT2CP102472BD1339. Said kit can easily be had for under $190 USD shipped if you look around.
The OS lives on a 2GB Sandisk Cruiser on the internal USB port. The orange pulsing glow from within is a mismatch for the green and blue lighting, but it works reasonably well, and the image doesn't benefit from anything larger.
I pulled the 250GB drive the host came with and dropped 4x 3TB WD Red drives in a RAIDZ1. It seems to run just fine, though I found throughput a bit better off the 250GB Barracuda drive than I do from the 4x3TB drives in a RAIDZ1 config. I opt to rsync my critical data to another local host hourly, and would personall opt for RAID-10 before RAIDz2 for throughput on this config, but I wanted the extra storage of a RAID-5 style config.
Only gripes about this hardware would be USB 2.0 only ports (add-in card can solve) and SATA-2 vs. SATA-3 (again, an add-in card can solve). The NIC may be a bottleneck in some cases (another add-in card), but given you only get 2 PCIe slots (1 x16v2 and 1 x1), you'd have to pick and choose your upgrades. Given that I saw 900+Mbps when populating the server w/ ~1.8TB of data (USB 3.0 external from my desktop using gcp under Ubuntu over CIFS), I don't think you'll find too many bottlenecks. The CPU is a bit under-powered for things like real-time HD Transcoding under Plex (currently required for Roku subtitles :-/ ). I kinda wish each drive tray had status and activity lights, but at the budget price, it's fine w/o.
Overall, it performs much better than the D-Link DNS-323 it replaces, while still remaining reasonably power-efficient.