Running one card you will get 16X speeds as that is on it's own 16X channel. If you run SLi, on that board, your two cards will share that 16X channel, resulting in each recieving 8X bandwidth. On one of those expensive boards you will get two 16X channels, one for each slot resulting in each card recieving 16X bandwidth when running SLi.
So what does this all mean???
Not a whole lot. Two current cards cannot exceed the bandwidth of one 16X slot, so for now there is no benefit two channels. Future cards are said to take more advantage of the two channel SLi motherboards, so maybe some day it will help.
As to SLi over all, not worth it. 2X the price for 1.3X the speed, sometimes less. Two 7800GTs will cost more then one 7800GTX and the 7800GTX will be faster.
Bottom line, your 7800GT should serve you just fine, I think you will be happy with the results. I know I'm very happy with my 7800GT. Forget about SLi, atleast until the technology improves.