256mb GPU X SLI=512mb?

If a notebook has two 256 SLI enabled GPU's, does that mean it is equal in memory to a 512 gpu?

 

In memory yes, but the two GPUs will perform much better than X1 512MB as there are two of them.

 

No; you still have 256MB of effective video memory because the data is mirrored in both cards.

 

Hmmm, then how much of an improvement is 2 GPU's over 1 GPU of the same model?

 

It depends on the game and the settings. Video memory isn't a big issue - anything 256MB and up these days is perfectly fine.

In my personal opinion, while I think SLI laptops are very impressive, you're better off buying a desktop for gaming and a cheaper laptop for on-the-go unless you want an all-in-one device.

 

Well the laptop I'm thinking of getting is the M9700, I was thinking of the m9750 but it's too far out of my budget. I'm just worried about the m9700's CPU, and I'm put off by how much shipping is from Alienware.

 

The answer is somewhere in between. A lot of data has to be mirrored between the two cards, but maybe not everything. It depends on the game you're running, as Chaz said.

The problem with SLI is that an awful lot of things go on under the hood, and the method in which it works varies widely from game to game. (Some games let each GPU render alternate frames, which is fairly efficient, and can give close to twice the performance. In other games, that isn't possible, so they have to fall back to different methods (such as one GPU drawing the upper half of the screen, and the other handling the lower half. Less efficient and harder to do properly, and requires a lot more communication between the two cards, but it always works)

In general, I'd say a SLI setup is ~30% faster than a single GPU. This is just a rough estimate based on no actual benchmarks whatsoever.
Unless you have no spending limit, you're better off without SLI. It's just not cost-effective. It may be necessary to get the absolutely highest performance, but to get a good performance/price ratio, stay with a single GPU. It's only if you want something even faster than the fastest single-GPU solution, and you're still not worried about price, that you should consider SLI.

(By the way, don't bother with Alienware. You pay double price, and all you get (apart from some questionable customer service) is a neat logo on the computer. Not really worth paying that much more for.

 

Ok thanks for the info, I was considering Alienware because it's the only manufacturer I could find with SLI, but after what you said about SLI I will probably go with the ASUS G1S

 

Remember that a video card setup in SLI will NOT double the performance of the single card. You will be lucky if you get a 50% increase in performance out of putting the second card in there.

I second what Chaz says. Buying an SLI laptop is a bit of overkill. You can equip a similar desktop configuration with a single graphics card that will outperform the laptop SLI configuration for thousands less.

 

Rock Direct do at least one SLI laptop:
http://www.rockdirect.com/viewNotebook.php?pName=XTREME%20SL PRO

I had heard that Falcon Northwest did too, but their site is crap so I'm not sure:
http://www.falcon-nw.com/

Also what gfx cards are they? It would be madness to invest in a high end DX9 setup with DX10 cards just becoming avaliable.

 

A single nvidia 7950 GTX > 2x 7900 GS.

The 7900 GS setup in SLI only beats the single 7950 GTX in high resolution. Get the Dell XPS M1710, the m9700 is way too pricey.