300 res? Is that the dpi you're rendering? Higher dpi is not the answer and will have precious little effect on any image you make except to make the picture larger. In fact, anything over 72 dpi is overkill for raster images meant to be displayed on any monitor. 150 dpi is standard for print media, 300 dpi is used for expensive art books showcasing high-end colour plates. It has to do with the way in which our eyes interperate color information from different sources. Of course if your main objective is to make huge wall posters of your 3d renderings... Bump up the dimensions of the beast to 5 feet by 8 feet, but there is no need to go above 150 dpi. As for realism... Textures, lighting, and shadow maps are the most important things to consider here, I do believe. How do I make my hi-quality texture look great? Add some "bounce" lighting. Subtley coloured lights that just glance of the edges of your subject. Get rid of the Omnis and add some spotlights. Bumping up the Shadow map size from 256 to 1024 will help get rid of blocky shadows as well. Experiment and good luck!