OK I dont know how this step up program works but if it's: you get a 280GTX for $100 and you trade in your 9800GTX... here is what I think...
Look at it from a financial decision perspective only... as this is an investment decision... the $350 you already spent on the 9800GTX is a none issue its is what we call a sunk cost... so writte it off... it is not relevant...
Now what you have to decide is:
A) spend $100 for a 280GTX that sells for over $600
;
B) spend $750 for a pair of 280GTX (1 for $100 and the second for $650)
;
B) spend about $450 a single 260 (or $900 if you go SLI 260's)
;
C) Buy a single ATI4850 ($200) or a pair ($400);
D) Buy a single ATI4870 ($300) or a pair ($600); or
E) Wait and buy a ATI4870X2 that should sell for $500 or so
What is the resale value of that 9800GTX... probably not that great but lets assume you can get $100 for it (it sells new for about $200 now)... then
A) spend $500 for a 280 ($600 retail less $100 for your card);
B) spend $1100 for a pair of 280GTX ((2x$600)-$100);
B) spend $350 a single 260 ($450-$100) or $800 ((2x$450)-$100) if you go SLI;
C) Buy a single ATI4850 ($200) or a pair ($400) less 100$ for your 9800GTX;
D) Buy a single ATI4870 ($300) or a pair ($600) less $100 for your 9800GTX; or
E) Wait and buy a ATI4870X2 that should sell for $500 or so ($400 if you sell your GTX for $100)
If my understanding of the step up program is good AND you want to stick with NVIDIA BUY THE 280GTX... guys that tell you different considered the cost of the 9800GTX in the equation which IS AN ERROR on their part...
The ATI card on the other hand could prove interesting if your option was to SLI the 280GTX for $750 as the ATI 4870X2 may be a better bang for the $$$$. If you go with a single 280GTX their is no better deal than the step up program.
Finally my analysis assumes that you have the PSU to run the 280GTX and that you will not incure more cost to run the 280 then the 260...
Finally sorry for the so so English... I'm French...
Hope this help,
If you go the 260 route and buy 2 of them and SLI... you are out of the money... get my drift...