The way I understand SLi, you don't get scaling on the VRAM since the driver mirrors the data in both cards. Would this mean the GTX295 would likely perform similarly to adding another GTX260?
yes, though it has 240(x2) scalar units instead of only 216 on a GTX260
http://www.behardware.com/articles/748-8/report-nvidia-geforce-gtx-295.html old review though
Also, I really find the 4th monitor useful for displaying ventrilo/web/twitter/whatever while gaming on the TH2G. The last time I had SLi was two 8800GTS 512's, and back then you could not have an AGP outside the SLi matrix for 3d apps, meaning the other monitor would go black while I was gaming. Is SLi still setup this way? If so, would it affect the GTX295 the same way since both processors are on one board?
yes SLI does support only one output
and yes a GTX295 is using SLI
(and it's not hidden like some ATI X2s drivers try to hide CF...)
the only useful point of a GTX295 compared to a dual card solution is the single PCI-E connector.