16x4x isn't 20x or 2x10,when a motherboard supports crossfire at 16x4x mode it means that when you put 2 cards in it in order to crossfire them,one of them will operate at 16x speed and the other one will operate at 4x,and for gaming this is not recommended,go for either Dual 8x or Dual 16x if u want to CrossFire http://www.tomshardware.com/forum/245454-33-crossfire-f...
"A note about Motherboards:
Some Motherboards support Multi-GPU Technology at Dual 16x mode, some support it at Dual 8x mode and some support it at 16x 4x mode Etc, what are the differences?
Well for getting the best performance out of Multi-GPU cards,you need a Motherboard that supports Multi-GPU at dual(Or more) 16x mode like Nvidia 790i ULTRA SLI, Nvidia790i SLI, Nvidia 780i SLI, Nvidia 680i( As i said, NVIDIA 790i ULTRA SLI ,790i SLI ,780i SLI and 680i SLI support 3-Way SLI too(Nvidia 680i LT SLI chipset has 2 PCI-E 16x slot which supports 2-way SLI only.),Nvidia 590i SLI, Nvidia nForce4 SLI 16x, ATI CrossFire Xpress 3200,Intel X38,X48 and X58.
Dual 8x is a very good configuration too,it doesnt have alot of difference with Dual 16x and its cheaper too.
Chipests like Intel P55*,Intel P45 Nvidia 750i SLI, Nvidia 650i SLI,NVIDIA 570i SLI, NVIDIA nForce4 SLI, ATI Radeon Xpress 200, Intel 975x and ONLY few Intel P35 boards(Like ASUS BLITZ FORMULA and ASUS BLITZ EXTREME) support Multi-GPU at dual 8x mode.
However for cards like radeon HD 48xx series,its better to get a Dual 16x motherboard because it performs better.
However,there are some expections too,for example EVGA has a 750i Motherboard(The model of it is
EVGA 750i SLI FTW)Which supports SLI at dual 16x mode unlike many 750i Motherboards which support SLI at dual 8x mode.
The last one is 16x 4x, this one is the weakest and doesn't perform very good compared to dual 8x or dual 16x,so i won't recommend it for Multi-GPU configuration. Intel P35 and P965 chipsets support Multi-GPU at 16x 4x mode.
Another notable pair: the P55 (socket 1156), which supports one x16, one x8, and one x4 - compared to the X58 (socket 1366) which supports two x16, and one x8... I tend to think of the comparison like this: the X chipsets are like a chainsaw - if you want incomparably brutal speed, but can do without a lot of fine "tweaker's" adjusments; while the Ps are like a scalpel - they don't support the overall subsystem speed you can get from the Xs, but you can fine tune the memory to "within a picosecond of it's life", with excellent control, right down to the seperate DIMMs. As reviewed above, there is very little in the way of graphics cards on the market that will 'overrun' even a x4 slot, much less an x8 (perhaps the dual-GPU 4870s, or the new 58xx cards, especially in dual GPU config), but, until about six or eight months ago, the best use for a x4 slot was to throw a hardware RAID card in it. While there are still x4 RAIDs available, about half a year ago or so, pretty much all the good ones made the transition to x8 - and it payed off handsomely...
Buying a 4850 won't allow crossfire to your onboard video if thats what you may be thinking. The 4200 series onboard will only allow crossfire x to a 3450 series or similar card. This is handy if you have one lying around, if not the 4850 will be much more powerful in anycase. Also since your thinking about crossfire and you don't have any dedicated video cards thus far it's good to know one higher level video card is enough for most everything.