For rendering it is easy, the more money the more power.
The more cores, the higher their speed, the more memory, the faster the memory the faster the rendering.
It scales pretty much linear.
Downsides is the filling of Octree and some Comositing stuff, which is not multithreaded (yet) but rendering itself is.
It also depends if you are GPU or CPU rendering…
But there is a ratio where the cost does not justify the performance gain anymore.
If I had to build a renderfarm right now, I´d most likely choose:
AMD Athlon II X4 630, 4x 2.80GHz ~ 80 Euro
Generic AM3 mainboard ~ 60 Euro
8 GiB memory ~ 120 Euro
64b Linux in VM bootet over network running dr.queue or loki
should consume ~150 W, so I buy it 4 times which is about 600 W consumption, so with a nice 850W PSU for ~130 euro you´r good to supply all 4 if you got a good 12V Rail, you got to know though what you´r doing because of leakage current supplying 4 boards with one psu.
Totals with ~1200 Euro, excluding network cables and some casing, but you can simply put the mainboards on 4 threaded bars holding them in place with 8 nuts a boards and put them in a corner somewhere 
or into a Helmer
Bottomline with some OC’ing you get ~50GHz renderpower for 1200 euro.
For stills it is pretty useless unless you render tiled, but for animations it saves time galore.
Lets say, your current assumed dualcore with 2x2.8GHz, 2GiB ram, 32b needs ~5 minutes for one frame.
Your animation is 1 minute with 30fps. So your puter would render simplified:
53060 = 9000 Minutes = 6,25 days.
8 GiB memory, 64b will most likely boos the performance up to 4minutes a frame.
The quadcore technology will most likely give you another 15 seconds.
so you´r down to 3.75minutes per frame.
now with 4*4 cores you can render 4 frames at once in half the time.
so the quadcore cuts it down to lets simplyify, to 2minutes per frame.
4 frames at a time.
2minutes * 30 fps * 60 seconds = 3600 Minutes
You render 4 frames at a time, so 3600/4 = 900 Minuts.
Same job done in 15 hours instead of 6.25 days.
Building a renderfarm is not about brute power though, its about the best price/GHz or MFlops for that matter.
You got to weigh the prices carefully.
Rather buy 2 slower Quadcores with 2 Mainboards - as it can be cheaper/equal or outprice only by a small amount - as the fastest Quadcore there is with its mainboard and still give you less power.
Lets take the PhenomX6… six cores, 3GHz… it is 18GHz for guessed 1000-1200 Euro. ONLY the cpu.
As listed above, same money buys you 50 GHz
And as you do now, rendering several files on your computer at once… It is slower than rendering them one after another.
Lets assume you got a Quadcore. 1 File rendering in 8 threads on 4 cores can utilize the processors cache as there´s lots of similar data, same goes for the RAM.
If you got 4 files, the overhead of the threads and the intesive memory usage makes the whole thing slow down.
On a historical note…
Babylon 5 the early episodes where rendered with a cluster of 25 Megahertz Commodore Amiga/68020.
Cheap and enough of them got the job done… =)
HTH.