Roberto,
Colour is right. If you ask this question to five people, you'll get ten answers! :P
I'll share my opinion, which will add confusion since I'm going to disagree with Colour on a few points. We do agree on a great many things, but not everything.
GRAPHICS CARD BRAND - I agree with Colour on this. Unless the upcoming iClone6 benefits from CUDA, the ATI cards can be an excellent choice. My son models with Blender, which has a "Cycles" rendering engine, which can use CUDA, so for him Nvidia is an obvious choice.
GRAPHICS CARD MEMORY - I would recommend a minimum of 2GB GPU ("graphics card") memory. I'd recommend shopping for a card with 3GB, or even 4GB if possible. Why? Because you can load more objects (props, avatars, terrain, etc.) with larger texture maps if you have more GPU memory. The models will only increase in size and complexity, so you'll be using more resources two years from now, so always round up.
GRAPHICS CARD CONFIGURATION - A lot of gamers run dual graphics cards. Both ATI and Nvidia have improved that capability over the last few years, but generally you'll be less likely to encounter any instability or incompatibility by running a more powerful SINGLE CARD that if you try to run a dual card configuration (cross-fire for AMD, or SLI for Nvidia). Colour alluded to this also, so we agree on that point. So I recommend a single-card.
GRAPHICS CARD PRICE/PERFORMANCE - The top-end graphics cards mostly allow gamers to push a huge amount of pixels with game quality settings at their highest levels. Even for a gamer, spending over $350 USD does not help you if you only have a single 1920x1080 monitor. But if you have two or three higher-resolution monitors, and you're a gamer, then you might wish for more graphical horsepower. For iClone, I'd think the $250-350 range is a sweet spot, and as I mentioned above, I'd put a premium value on the amount of RAM on the card.
Here is an article summarizing some graphics cards at various price points.
SYSTEM MEMORY (RAM) - For a "mainstream" PC, I would start with 4GB of RAM. For anyone doing "content creation," be it iClone, Blender, video editing, or even heavy Photoshop work, I would immediately increase my recommendation to 8GB. I'm running with 16GB, which was a bit of an indulgence, and frankly I don't think I've actually used more than 6 or 7GB yet, but I decided to get it while I was building my computer.
SSD - We (I) have talked about this a lot lately. I am a total fan of SSDs, and recommend a minimum of ~250GB for your C: drive. Even that will go quickly. You'll need a second drive, and I recommend at least a 1TB hard drive. Given their prices, I'd suggest a 2GB second drive.
One user (Rampart) has been struggling with long load times, and his SSD was getting a lot of the blame, but I believe there are other forces at work. We did some tests, and his SSD is 3 to 30 times faster than his HDD. You can read all about it, including his benchmarks, in THIS THREAD.
CPU - Simply put, faster is better. For heavy users like us, the Intel i7 processors are pretty much the default selection. For casual "web surfers" you can trade between Intel and AMD at any given price point, but Intel has dominated the enthusiast market for several years now.
OPERATING SYSTEM - I have to lean toward Win7. I haven't used Win8 much, and frankly I'm not fond of it. But this is 99% an emotional preference for me. I can't say Win8 would be less stable or anything like that. Win7 will be around for a long time still, so it should be a safe bet. 64-bit, of course.
OTHER - A high-quality power supply and good cooling are also important for overall system stability and longevity.
FINAL COMMENT (for this posting) - I generally disagree with Colour's comments that excess power can lead to issues. Generally speaking, in my opinion, you're far more likely to have issues with an under-powered system.
Sadly, budget always comes into play, but you should be able to buy or assembly a solid system for $800 USD. Especially if you build your own, you can recycle your case and your DVD drive(s) and stuff like that. Once you hit $1,200 any additional cost will probably to unnoticed in terms of iClone performance.
iClone 7... Character Creator... Substance Designer/Painter... Blender... Audacity...
Desktop (homebuilt) - Windows 10, Ryzen 9 3900x CPU, GTX 1080 GPU (8GB), 32GB RAM, Asus X570 Pro motherboard, 2TB SSD, terabytes of disk space, dual monitors.
Laptop - Windows 10, MSI GS63VR STEALTH-252, 16GB RAM, GTX 1060 (6GB), 256GB SSD and 1TB HDD