choosing a graphics card

Yes you can use an ATI card on a SLI motherboard, but you can't use two in crossfire since they're not compatible with each other. I would suggest an HD 3870 card if you can find it for $200-220. If not, then you'd just have to wait till they become more abundant. If you can get a good deal on the 8800 GT, I would go with it too. Depends which one you can get for cheaper (probably the HD 3870).

More video card memory kicks in at higher resolutions only, you won't notice a difference at reasonable resolutions. Core/Memory/Shader mhz and the memory bus are what matter.

I'm assuming that pci 2.0 works in pcie16 slots, but my motherboard limits each pcie slot to x8 speed because it's for sli. Someone told me that that won't matter at all because graphics cards right now can't fill it up. Is that true? If I get a 3850, would it be slowed down because of the motherboard?

Im probably going to confuse you even more now (sorry) but its not quite as simple as below.
Before i start i would like to stress that im not saying Nightscope is wrong just trying to elaborate a bit.

NightScope
More video card memory kicks in at higher resolutions only, you won't notice a difference at reasonable resolutions. Core/Memory/Shader mhz and the memory bus are what matter.

While this is generally true i felt it important to point out that a lot of the newer games are using a lot of textures and this is another area where the extra ram comes in handy, If i was buying a new Graphics card today and was planning on keeping it a couple of years at least i would get 512 as a minimum to allow for future increases in game requirements,it was only a couple of years ago that 128 was considered the norm.
Also the Core/Memory/Shader mhz and the memory bus advice while again is a good rule of thumb only really works when comparing like for like,, say one make of gt to another or two cards within one series as long as they are built on the same architecture. if you take the old 8800GTS and the 2900XT you will see that on paper the 2900xt should be the better card but it wasnt. This is an example of what i am saying about the card being built differantly, its a bit like comparing apples to oranges it just dosent work.

Chipset Manufacturer ATI
GPU Radeon HD 2900XT
Core clock 750MHz
Stream Processors 320
Memory
Memory Clock 1650MHz
Memory Size 512MB
Memory Interface 512-bit
Memory Type GDDR3

Chipset
Chipset Manufacturer NVIDIA
GPU GeForce 8800GTS
Core clock 520MHz
Stream Processors 96
Memory
Memory Clock 1600MHz
Memory Size 640MB
Memory Interface 320-bit
Memory Type GDDR3

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