SLI versus single card?

[Commander_Fett]

I'm planning to upgrade some of the specs on my comp, and I've been having trouble deciding what to do for my graphics card. I have decided on the option of getting an nvidia GeForce GTX 465 or 2 sli linked GTS 250s. It is my understanding, that 2 GTS 250s would be better, since each would only have to render half the screen, and their combined power well exceeds the GTX 465. However, in some places people say that you get a much smaller boost in preformance, especially at higher resolutions (I'll be playing on 1900x1200), while others say it's 200%. Does anyone know which option would be better?

EDIT: it is worth note that I will be using Windows 7, and if I go for the gtx 465 I can get the superclocked edition. Also, I may be able to get the GTX 470, if the improvement over sli gts250s or a gtx 465 is substantial enough to warrent the extra hundred bucks.

[Teancum]

The benefit is never an even 2:1 ratio with dual cards. I'd say you'd get 150-175% boost most of the time. For me it's not really worth it, particularly if you're not going to upgrade the whole system.

[Commander_Fett]

I'm getting a new motherboard and ram as well (uping from 4 gigs of DDR2 800 mhz to 4 gigs of DDR3 1800 mhz)