ECS P45T-A Black Series, SLI and CrossFire???

It's only crossfire compatible. It will be able to run the 9800gx2, but not sli it. When in crossfire, it will put both slots at x8 instead of x16. This won't make a difference though, because 1) x8 2.0 is the equivalent of x16 because 2.0 has double the bandwidth and... 2) You won't see any difference in performance running x16 instead of x16 2.0. The only card that might be limited by bandwidth would be the 4870x2. The P45 is a good chipset, but I'd recommend getting one from a different company.

which one would you recommend?
I see that the ASUS P5Q Pro seems to be a popular choice at newegg.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Yes, the P5Q Pro is a good choice.

What is better 9800gtx+ sli on ASUS P5N-D nForce 750i (which I believe has both lanes at x16 even when in sli, correct me if I am wrong) or his iceq4 turbo hd 4850 crossfire on ASUS P5Q Pro(x8/x8)? I mean gaming performance wise. I think the P5Q Pro might remove any performance advantage the hd 4850 might have in crossfire over the 9800gtx+ in sli.

750i
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

P5Q Pro
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

EVGA 9800gtx+
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

HIS Iceq4 hd 4850 turbo
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Note: The HIS hd 4850 turbo I listed is not the same as the turboX. The only difference is the turboX has an overclock while the turbo sticks to stock speeds. Both the 9800gtx+ and the hd 4850 are $160 after rebates but even if the prices change I want the focus to be on performance.

What games are you going to be playing? One might be better than the other based on what games you're playing.

FPS and RTS. Examples Crysis, Far Cry 2 when it comes out, World in Conflict, Empire Total War when it comes out, etc.

But my question is more general. Wouldn't the 750i be more similar to the x48 or x38 with its pci-e 2.0 x16 in both lanes even in sli (please correct me if I am wrong)? So on average the sli build would be better.

LOL. Newegg changed the hd 4850 to a overclocked version and increased the price but still kept the same html. But lets still talk about it as if it was at stock speeds. HD 4850 crossfire on p45 vs. 9800gtx+ sli on 750i.