i was gonna type a huge essay to explain but i decide not to because if i do, it'll be 100 pages in length
hardware wise -
in a nutshell here's what you need to know. l2 relies heavily on ram, your storage device (hard drive or ssd etc), and cpu
ram: higher frequency and lower timings = better. frequency will yield better results than timings unless it is set up extremely poorly. oh and make sure you have enough to run all the things in your background. 4gb should be more than enough
storage: the faster the better. put your operating system, game client, and virtual memory on the fastest drive if you're using a ssd. otherwise put them on different partitions (or another hard drive on the comp)
cpu: despite what everyone seem to believe, MULTIPLE CORES DO NOT MATTER IN L2. l2 runs off the UT2 engine, that thing is so old, it can't even take advantage of hyperthreading, let alone multi-core processing. quad cores actually run SLOWER than dual cores if both cpu's have identical specs and speed. what you're looking for in a cpu for l2 are: 1. raw speed and 2. cache size. raw speed is self explanatory. as for cache, you want quantity, not speed. the bigger the better.
software wise -
l2, since the game first came out, has HORRIBLE memory leak issues. memory leak is when the data in your ram does not erase itself when it is supposed to (something that can easily fixed with 1 line of code, fuck ncsoft). combine that with the constant data swapping between ram and page file, well basically the longer you leave your client running, the shittier the performance will be. this is nothing a simple client restart won't fix
little tips and tricks on the software side are primarily memory management and slight network alterations. personally i wouldn't do it unless i know what i'm doing, because the slightest mistake has the potential of fucking your entire computer up.