4GB RAM is rather paltry for modern computers. While Rhino does not take up much RAM while modeling, while rendering, it can more than double. If the hardware RAM runs out, and you run into virtual memory, your machine will be slowed greatly. I recommend 8 GB for small to 16 GB for moderate projects, and then you should be able to shut off Windows virtual memory altogether, for it does not use it wisely
[A Mac with a SSD should have 16GB for CAD or even serious browsing, as OSX will cross the 8GB line, even while even surfing sometimes, and not be shy about using the SSD for virtual memory, which will greatly accelerate its wear.]
Small computers have it hard as far as cooling. A machine that small may not like to raytrace all day, and/or it may not be very quiet while doing it. Look for 120mm or larger case fans.
IMHO, multiple CPU machines can be somewhat temperamental. They rule for raytracing, but are often beaten for design because of their fast single-core performance by something like a $320 i7-4790K chip.
The Xeon processor in the machine in the small case is a quad, which is fine for few renders a day. You may want more cores for a lot of raytracing, but the next step up would be something like a 8-core Intel Core i7-5960X, which is $1000 for the chip alone. There is a mini ITX X99 motherboard for it. This is a wonderful kind of insanity., but I’ll wait for MSI for make one, while I am accepting donations for a i7-5960 : )
If you only need a few raytraces here and there, a mobile workstation may work out for you, such as a Dell Precision or a Lenovo W541.
[I use a W540 laptop for Rhino, and I use a desktop for raytraced rotated animations using Rhino’s stock renderer. Four years of constant effort has made the new Intel processor about 15% faster than my 2600K, while a new $320 video card is about 220% faster than my GTX570.]
Some raytracers such as Arion use the GPU for raytracing, and this will be the way the wind is blowing as for your dollar, video cards generally have ten times the FLOPS than CPUs, but getting that to use in a small memory window can apparently be challenging.