You still haven't said what you'll be using these for. If its for games 4.0 is all the further you need to go. If its primarily going to be for music 4.0 is not going to help and is just going to mess up the sound stage, so stick with 2.0 or 2.1.
As others have said adapters are fine, and balanced cables are really only benficial for longer runs. If you are planning on sticking with onboard audio you can get 1/8' to XLR. I'm going to be doing something similar with some KRK monitors only I'll be using a external DAC so I'm going from RCA to XLR using Cablematters cables. They are pretty affordable and the quality seems really nice so no complaints. Here is the
1/8 to XLR version you'd want for onboard audio, ohterwise if you think you might upgrade to an external DAC you might want to get the longer cables in RCA to XLR and get a 1/8 to RCA adapter.
What would I need? A receiver with pre-outs? I was thinking, I'd get this sub from Monoprice:
https://www.amazon.ca/Monoprice-10-Inch-Powered-Multimedia-Subwoofer/dp/B00TZBBZV8/
I'd really suggest getting a better sub, you have some decent monitors there so you might as well match up the quality with the sub. I have the 10" JBL which I can honestly say is insane overkill for my office bedroom so I'd probably look for an 8" from any of the major players. A JBL, Adam, Yamaha sub is also going to have way better resale value than an off brand like Monoprice.
*edit*
You
shouldn't need anything other than your onboard audio, for sure if you go 2.1, though 4.1 could get complicated. If you go 2.1 you can run a straight up stereo signal to the sub and use the subs built in crossover (this is what I"ll be doing in my setup). If you go with 4.1 You'll have to look into what your drivers are capable of as you'll have to run a dedicated output to each of the 5 channels and hopefully there is a software crossover.
Honestly I'm a big advocate for two channel stereo audio and just getting the best mains your budget will allow. Its much easier to setup, there are less room dependencies, and unless you are really into being "surrounded" with special effects in action movies two $400 main speakers are going to sound way more impressive and immersive than four $200 speakers.