|PCI-E really just comes in one format, which is X16 (twice as the AGP) unless you are using to PCI-E in SLI mode, which puts both cards in X8 mode. SLI is a new format offered on certain motherboards which allows two identical PCI-E cards to work in tandem. It's expensive having to buy two cards, but framerates are unbelievable.
I have an A8N-SLI Deluxe from Asus with SLI. Incredible motherboard.
Hope this helps,
not entirely true..
pci-express bus is a four-wire link between only two devices called a lane. The two pairs of wires carry single bits of data in each direction.
the smallest and slowest link is called an x1 (pci-x1) link, which has a single lane and which transfers one bit of data at a time in either direction. an x2 link has two lanes and transfers two bits of data in each direction at a time, etc. the standard sizes of PCI Express links are x1, x2, x4, x8, x16, and x32, with the possibility of the future development of an x64 link. An x32 link has 32 lanes and so can transfer 32 bits of data in each direction at a time. Currently motherboards only use x16, x1, and x4.
since the length of a PCI Express slot depends on the number of lanes it contains, an x16 slot is about the same length as a PCI slot, but x8, x4, x2, and x1 slots get progressively shorter as the number of lanes they contain diminishes.
since each end of a PCI Express bus link can negotiate how many of the available lanes will be used to transmit data, an x1 adapter card can be plugged into any of the larger slots. basically as long as the card you're using is for a slot smaller than the one you're plugging it into, it'll work; you can plug an x1 or x4 card into an x16 slot.
each x gives you about 500mb/s worth of bandwidth. example: x1 has 500mb/s, x2 has 1gb/s, x8 has 4gb/s, x16 has 8gb/s.
personally i'd not settle for anything without sli (or at least an x16 and a separate x4) at this point in time if i were buying in preparation for the future. a year and a half - 2 years from now you'll be using physics processing units along with your graphics processing units to run games so you'll need the slot/bandwidth.