Don't let the memory size fool you... bigger isn't necessarily better anymore. Companies like to MARKET larger memory sizes, but what they don't tell you is the LARGER memory is also SLOWER memory. Plus, Aces High will not even touch the 512MB, so going to 1GB is overkill.
DDR3 is MUCH better than DDR2. The Architecture of the 9800 GTX is MUCH MUCH better than the 9500 GT. I personally have a 9800GTX DDR3 'OC' version and I can promise you, it's nothing short of.. WOW. My GF has a 9500 (GT I think) and it seems to do well enough, but still can't touch my 9800.
For some reason, I'm seeing great deals on the 9800GTX lately (I got mine last month for $100 after rebate)... maybe because they've released the next generation NVidia cards...
Just make sure your Power Supply is good enough to handle the 9800....