I upgraded one of our machines with this graphics card. It's essentially an nVidia GTX570. A card... for gamers. This machine was previously running a 460, which compared to most 3D people I've come across, was quite good.

So, why bother upgrading your video card? [Important note: I don't spend money unneccessarily, I don't gain anything from the experience of owning nice computer hardware - it's just a tool].

Render Times

It will not improve render times. Cinema 4D's 'full' mode rendering, which is the typical output mode giving full visual quality, is processed via complex maths on the computer's CPU. It is NOT affected by the graphics card in the machine which is doing the rendering.

(An aside...)

More recently I've started outputting hardware renders to show clients how I see an animation developing. They look essentially like your editor view (so, very rough visual quality), and will scare your clients to death because inevitably they will be unable to mentally separate the 'motion' from the 'look'. It is good, however, to try and align your client with the same process you have to work through to produce their animation to avoid having to build every single preview and potentially post-produce over and over again in order that they can easily digest what you are showing them.

Hardware renders are very fast to produce, and they are done using the video card's hardware, so your graphics card will affect things here. However, they are typically so quickly rendered on almost any modernish card that the time saving benefit is negligable.

If you want to speed up your renders, you need CPU power, preferably multiple machines on a net render - it is the biggest crime in the world to be trying to do you day's work with a big render chugging away in the background on the same machine, slowing your computer to a grinding halt and making your workday less efficient. Your time is more valuable than CPU time. At the very minimum get a second box to use as a dumb render box which can be left on 24 hours working its way through a queue, then when you are not working at your main computer, run the net render client on there too to get it helping out with the rendering overnight.

Editor Use

Where a better graphics card does offer improvements is in your editor view. When you are sitting there, spinning round your 100,000,000 polygon scene and the whole thing is degrading itself to box outlines, or (the most irritating thing EVER), just sitting there seemingly doing nothing for 30 seconds after you try to open a scene while it processes all the polygons... That's your graphics card's limitations showing.

The other thing is often playing back animation in the editor will be of low FPS, in complex scenes with the 'All Frames' option on, C4D can reduce your frame rate as far as it needs, often less than 1 frame per second. Useless for assessing timing and feel of your animation. You can disable 'All Frames' and C4D will try its best to maintain real-time by skipping frames, so you end up with a slightly more accurately timed, jerky animation to watch.

These are things which get in the way of your day... All day, every day, your graphics card is failing you, limiting you.

Cost Effectiveness

If you are just starting in business and aren't busy, it might be fine to piss about wasting time being strangled by your own hardware. Your budget for upgrades is possibly nothing. If you are 'lucky' then your thirst for working on more complex scenes will be slower than those asked for by clients (i.e. you'll be getting paid for less complex work which you can do efficiently enough on your existing hardware).

However, the moment you have to regularly work a longer-than-normal work day, feel consistently anxious about looming deadlines, or start noticing you are waiting round for the computer all the time, it is time to streamline your operation.

I charge 'a certain amount' per day. While it might seem a good idea to be cheap, because obviously all decisions are made on price, so you'll get more work (that's sarcastic by the way), bear in mind your overheads aren't zero just because you are working out of your bedroom on a computer you already own. You need to forward plan for your future business development, and factor this in to your pricing.

3D people need higher spec hardware than any of the other creative industries I know of. If you want to be good, or at least better, then you are probably going to need better hardware.

That's all I can write about this for now... My point is, never hesitate at upgrading if you sense your existing setup is costing you even 30 minutes of waiting-around-for-the-computer time per day.