Well, lets look at some fun memory bandwidth comparisons between the two:
6600GT 1ghz mem = 1000 * 16 bytes = 16 GB\s
9800 Pro 340 (680 DDR) Mhz mem = 680 * 32 = 21.76 GB\s
That is a pretty large difference, and although most of us have experienced that fill rate increases are better than bandwidth increases, this amount will be noticeable. The divide stays about the same OC'ed.
6600GT 1.15ghz mem = 1.15 * 16 bytes = 18.4 GB\s
9800 Pro 400 (800 DDR) Mhz mem = 800 * 32 = 25.6 GB\s
This increased bandwidth makes light AA/AF possible in the newer games, DC looks much better with this stuff enabled!
What about fill rate? Well, that is even easier to look at:
6600GT 500 Mhz Core = 500*8 = 4 Gigapixels - giga or mega? I guess it doesn't matter much, I'll just continue with mega, the numbers are what matter...
9800 Pro 380 = 380*8 = 3.04 Gigapixles
This is important for actually drawing the tanks, people etc, and in my experience the 9800 Pro @ stock was good enough for DC. Since you have water and all, lets oc these guys then!
6600GT 600 Mhz Core = 600*8 = 4800 gigapixels - this would be a really good OC on this type of card in my experience
9800Pro 475 Mhz Core = 475*8 = 3800 gigapixels - Under water this could be possible, I don't know.
What you have in this situation is nearly the fill rate of the 6600GT with much more bandwidth. DC will fly on the 9800 Pro and you get at least 2xAA/8xAF, which you may or may not get on the 6600GT. In all reality either of these cards are fine choice. Let us know what you decide on!
Nat