I have an Asus p5b deluxe with the latest BIOS (beta 1238). I don't think this necessarily applies to me as I don't think this board can do SLI but, let's say I could. If I put another 8800gt in a PCI-E x4 slot I am assuming that that card would be bottlenecked correct? If it was setup to use SLI that would mean the difference would be low correct? Or, would SLI not even work because of the 4x slot?
It might just be a price thing but why are boards made with only PCI-E x2 or x4 slots, why not just make both of them x16?
In addition, I have heard that some games don't really perform better with 2 cards. I don't understand this, I thought SLI or Crossfire basically made the cards run as one card, so why would games have to be specifically designed for this? I have heard you are just better off (in most cases) buying a pretty nice single card and you would be fine. For the money you spend a few years later to get another of the same type you could pay a bit more for a single card and get more of a boost. Of course, it seems like DirectX 11 is coming somewhat soon so all those DirectX10 cards everyone ran out to buy are going to have been purchased in vain (correct me if I am wrong about anything in this statement).
You are correct in that your board won't do SLI. It might do CrossFire with ATI cards, but you'll have to check compatibility.
As for whether or not you could use a x4 slot for SLI, yes, you could. The throughput would be limited (and thus a bottleneck), but it would still work.
Not all boards have dual x16 slots because the chipset on the motherboard doesn't have enough PCIe lanes to do that. If the chipset only has 20 lanes, the best you can do is x16, x4 (or x8 and x8). This x16 + x4 limitation was present on many of Intel's boards before the x38 came out just because their chipsets had limited PCIe lanes.
Note that for SLI to function, the work involved with processing a graphical scene needs to be divisible between two or more processors (normally, the game is not written with this in mind). As such, if the system cannot find a good way to split the processing for a game, a game might suffer when running in SLI mode because the system is spending more resources trying to split the load than it gains from running two GPUs at once. Usually, NVIDIA needs to release upgraded drivers to better support newer games. Very fews games today still suffer issues with SLI, most will see minimal to exceptional improvement.
As for whether to run SLI or a single card, its depends on what you already have. If you are starting from scratch, a single faster GPU is better (in my opinion) then running SLI with two slower cards. This is because of your upgrade options (in the future, you can buy a second 285, which will tend to be cheaper than a single faster card, and two 285's in SLI will probably outperform the single card too).
Look at the other example, say you already own one GeForce 260, in this instance, you are better off getting a second 260 instead of upgrading to a single 285. The reason is that upgrading to a 285 will give you maybe a 20% boost over a single 260, but at a 20% price increase over a second 260. But buying a second 260 might give you a 40% boost, and cost you less than a single 285.
In my mind, SLI only makes sense if a) you already own one card and want to add a second, or b) you can afford two of the currently fastest single GPU cards. If you can't afford two of the top, buy one and add a second later. If you already own one slower card, a second will probably be faster and cheaper than the current fastest single card. Remember, upgrading only makes sense if you get a reasonable increase out of it.
As for directX 11, there are so few DirectX 10 games, and these games still run in DirectX 9 mode, I don't see any reason to hold-off for DirectX 11, nor do I see DirectX 11 making DirectX 10 cards obsolete.
It would not bottleneck on a PCIe 2.0 x8 lane. It might see some bottleneck on a PCIe 1.1 x8 lane, but that's doubtful. Tom's did a review a while back comparing x1, x4, x8, and x16 lanes on PCIe 1.1, and x8 was pretty close if not the same as x16 from what I remember.