If it were up to me, I would definitely upgrade the 9800pro to the 6600gt on ONE condition:
You sell the 9800pro.
If you can sell the 9800 pro on ebay, for example, you can scrounge about 150-170$ from it. Newegg.com sells the 6600gt for about 210$. Price difference?
~$50. NOT BAD at all.
The performance difference is pretty big if you REALLY tax the card. For newer games at par with DOOM3, the performance leap from the 9800pro is quite staggering. For older games, the two perform almost equal (i.e. battlefield vietnam).
Still, with the 6600gt, you can turn on more eye-candy features (AA, AF) without losing as much fps as you would with the 9800 pro.