Many will say that it's not really, but if you get a high-clocked 7800GTX like a Gainward Golden Sample or an XFX Extreme Gamer Edition then the performance gap starts to widen a lot more - I think that's the point where the GTX becomes worth the money.
In real terms this is what it means: http://www.bit-tech.net/hardware/2005/08/11/bfgtech_xfx_7800gt/6.html
I'd go with 1 x 7800GTX; will still be faster than 2x 7800GT's in many applications and of course you can get another in future.
I disagree - the 7800 GT SLI performs better than 1 7800 GTX in most instances. The "SLI doesn't work in all games" isn't the case because you can make 99% of games work with SLI by making your own profiles.
Are you saying the 1x GT works better than 1x GTX, or 2x GT works better than 1x GTX? I say the latter?
2x GTs may be better in some cases than 1xGTX, but you've lost your upgrade path. I'd opt for 1x GTX, and then add another one at a later date. It depends, like som many things, on your budget. If you know you can't afford another GTX later, then maybe there's no point, but I can't help thinking that if you can afford TWO 7800GTs NOW, then surely you must be able to.