Officially, I have 6.1 due to my amp, but my audio card only puts out 5.1, so the rear central speaker is a blended signal of the two rear speakers as mixed by the amp. The speakers are arranged in about a 4' radius circle centered on my head, with the front central channel blocked by the screens and the rear center channel blocked by the seat. These are classic BIG speakers with 10" woofers rather than the harsh tinny-sounding home theater speakers and my subwoofer is a 12" Dayton Audio. I balanced the system with a u-mik calibration microphone and Room EQ Wizard.
Apart from Wreckfest, the subwoofer does diddly-squat in racing games/sims, because the usual racing games/sims have no LFE channel like movies and video games do. Consequently, I'm considering moving the subwoofer over to the TV area.
As to the surround capabilities, only rFactor 2 is stereo-only. Yes, you can check the box for 5.1 or 7.1, but there are no effects. The rest of the titles have at least a rudimentary rear channel effect, while others make full use of surround channels.
Overall, with the smaller circle of speakers we typically have for our computer rigs, I find the surround experience is best with 4.0 or 4.1. Since nearly all titles support surround effects, 4.0/4.1 will enhance it due to better separation of channels. With 5.1 and higher, the channels seem to bleed over and you don't experience the directionality as much. Note that you need to configure your sound card in addition to the amp if you cut back to 4.0/4.1.