The 9800 Pro with 128Mb is far far better than the 9600Pro with 256Mb.
Despite having 'less' on board memory, it uses its memory much more
efficiently - it has twice the bus width (256bit vs 128bit on the 9600
Pro - note do not confuse this with the memory amount) and it has twice
as many pipelines (8 vs 4). This means that it runs much faster than a
9600 Pro. The extra memory on the 9600 Pro will not be used at all
efficiently so makes it null and void
You can see a comparison of the cards in this table as well:
http://techreport.com/etc/comparo/graphics/
Note that the 9800 Pro has higher power requirements than the 9600 Pro -
you need to have at a minimum a good 300W CPU with at least 15A on the
12V rail (which you can find out by looking at the side of your PSU -
there will be a sticker with all the details)