I am just starting to think about upgrading my printer with V-Rail and eliminating the smooth rods/bushings/LM8UU horribly noisy bearings and when I am finished it should blow the current Prusa I3 Rework out of the water for speed, accuracy, noise, and most importantly REPEATABILITY (which the I3 Rework doesn't have without a lot of modding which I have done). These two pieces are the first ones I noticed I will need and for a 3d printer can use in RP (some you just need in metal and others you do not).
The source stl files I used for these came from http://www.thingiverse.com/thing:500041. His printer looks very nice and if I didn't already have mine I would probably be building his (if only this were 10 months ago).
http://www.openbuilds.com/ has more of this sort of thing with many more parts for you to make some really nice stuff with.
These two parts are structurally pretty important so I would use as much infill as I could.
Infill: Yes, 50-70%
Vertical Shells: 5
Horizontal Shells 5 top & 5 bottom