[alex]

SLI or Scaleable Link Interface was created by Nvidia.

It's basically the ability to attach two graphics cards to your motherboard to allow increased performance in games via a 'dongle' that connects the two cards internally.
Some SLI motherboards have 2 8x pci-e slots, some motherboards have 2 16x pci-e slots and to confuse you further some have a 16x and a 8x or a 16x and a 4x.

The first downside is obviously cost, second is the power needed to run two cards. (Nvidia's top-end card the 8800gtx takes upwards of 150w from your psu).

The third, and most important downside in my opinion is that not all games benefit from SLI as it hasn't been coded into them. So you could play your favourite game on two cards and not get any benefit from the second card whatsoever.

16x refers to the data-bandwidth for full speed pci-e. The maximum speed of AGP was 8x.

Regarding the sentence above read below for a post i placed previously, hope its of some help.

AGP (Advanced Graphic Processor) - Some history first, Back in the mid to late 90's the need for faster graphics acceleration led to the implementation of a new kind of interface from PCI, enter AGP. Most motherboards made since the late 90s include either an on-board integrated AGP gpu, or an AGP slot for a dedicated card, be this (now) either ATI or Nvidia. But 'back in the day' you'd see gpu's from 3dfx and Matrox.

The primary difference between AGP and PCI is; to load a texture, a PCI graphics card must copy it from the system's RAM into the card's framebuffer, (with a lower bit-rate memory controller),whereas an AGP card is able to read textures directly from the systems RAM.

Early AGP speeds weren't anything to write home about (by todays standards, running at 2x or 4x the speed of PCI). AGP really came into its own with 8x.

A breakdown:

AGP 2x
A 32-bit channel operating at 66 MHz double pumped to an effective 133 MHz resulting in a maximum data rate of 533 MB/s.
AGP 4x
A 32-bit channel operating at 66 MHz quad pumped to an effective 266 MHz resulting in a maximum data rate of 1066 MB/s (1 GB/s)
AGP 8x
A 32-bit channel operating at 66 MHz, 8x pumped, delivering an effective 533 MHz resulting in a maximum data rate of 2133 MB/s (2 GB/s).

And just as importantly, the voltage required had dropped from an initial 3.3v down to 1.5v.

Enter PCI-E (PCI-Express).

Long story short, PCI-E is basically just AGP 8x but the data-width has been doubled from 32-bits at once to 64-bits (hence 16x). This refers to the signal from the motherboard to the card itself NOT the gpu's onboard RAM.

Also, with this new type of graphics interface manufacturers were able to up the wattage from the motherboard to the card, and more power=higher performance.

Cards were now able to have wider bit-rate memory controllers onboard. Its doubtful you'll see any AGP card with a memory controller wider than 128bit, whilst most pci-e cards have 256bit,some even have 512bit.

The most important things to look for when sizing up a card as a new purchase?

gpu speed- The faster the gpu speed the faster the card will 'work the instructions out'

ram speed- Firstly, look for DDR3 (this memory is prevalent on most cards nowadays so i won't go into ddr2), then look for the speed the ram runs at.

Manufacturers will always quote an 'effective' speed of the ram. This isn't marketing hype, it simply means you've 2 halves of the ram amount, both connected separately to the ram controller on the card, running at a given speed and as they run in dual-channel at say 700mhz each, this will give an effective speed of 1400mhz.

Memory controller- As the most common cause of slowdown in games is the speed the memory 'processing' its data you want the widest bandwidth possible. As an example.

A card with its ram running at an effective speed of 1400mhz, and a memory controller at 128bit, with have an effective memory bandwidth of about 22gb/s.

The same effective speed ram, with a 256bit controller will have an effective memory bandwidth of about 44gb/s.

Ram amount: DONT be fooled into thinking the ram amount is going to make any (marked) difference to your gaming experience. 256mb is fine for any direct 9 game (obviously providing your memory controller bit-width, gpu speed etc) is fast enough.

512mb+ ram is only good if you've (1) an extremely high end card, (2) For textures at very high res', (3) For fooling the public at large with big numbers so they'll think they're getting a better deal and (4) If you're gaming at very high resolutions: 1680x1050 or 1900x1200.