I have a 1440x900 LCD and I still went for the 512MB 8800gt. Some games already require 512MB to set textures to high, and I don't see this trend reversing as more dx10 games come out next year. Spend the extra and you won't regret it.
Go with the 512MB dude , 8800GT is quite a revolution for its G92 Arcitechture, but if you strip it off all its memory it'll start being plain stupid.
I'm using 1440x900 , its not as little demanding as you might think, i was surprised to find it maye even less than 10-12 % more performance than 1600x1200 after running tests forcing that resolution.
My crysis is barely running on very high 2xAA (Only in multiplayer, I remove AA in camaign cause its slow as ****).
Don't buy a new card because it has dx10(.1) support. It's worthless. If you can play your games fine and don't need to, better hold your money and get a next gen card with better dx10 support. These cards are fine but they are going to drop a lot in price in the next few months, the new GTS will be out soon too, X2 cards with 2 gpu's on 1 board, the war is just starting.
It's also the big question, what will new games bring, more better sli/fire support? If the support gets better, 2 3850 cards become really nice for the price. Maybe 2 8800 GT's in SLI.
But my best advice is, if your card can run it without any problems, keep it and wait a bit longer. Do you need more then 60 fps in a game? Yes DX10 looks a bit better, but as there are still no DX10 only games I doubt it's worth the extra money to get vista and/or a new card.
Just my opinion but, I think you should just wait. I know it is temping to go out and buy a powerful card to push eye candy further but you do have the 1950pro. I mean, that should hold you off till lets say Jan-Feb and then you can buy the latest card or any other card as it should be priced a lot better then. Assuming you have money to burn, you can do as you please but judging by your e6300 and having 256mb in mind, you must be budget minded but its your call.