You want to play games at a native res of 1920x1200.
Heres what you propose:
1: Upgrade to a 7800GS (300USD) AGP
2: Upgrade to a 7800GT (300+USD) and buy an SLI board.(150+USD)
The answer is NONE. None of those cards will play games at that res so forget it. 7800GS, don't waste your time. 7800GT, don't waste your time either. The 7900GT is coming back into stock, is faser than a GTX and will handle most games at that res, but not all. Its cheaper, or the same as the 7800GT(depending on the model). Second of all, theres plenty of great SLI boards at less than $100, around the $90-120 mark. I don't know what board you saw for $150. But it was top of the line, SLI boards top off around $150-$170. If your paying that much for a SLI mobo then go with the DF1 lanparty which is around that price. or the ASUS top of the line SLI board.
Good LUCK!, Oh yeah almost forgot, 1800XT is GREAT! Its your choice.