well the best card under $200 is the ATI 5770 but your LCD does not have a demanding resolution. An ATI 4850 or Nvidia GTS 250 would be more then enough to hold you over until you feel like getting a new LCD. Both can be had for around $100 on newegg.
The 8800GTS 320mb will still be ok for that resolution but I can understand wanting an upgrade at this point. If you are looking for something to last years I would definitely want a DX11 card which basically means an HD5670/5750/5770. The HD5670 will be an ok upgrade from your current card but not impressively so. The HD5750 would be very good currently for your resolution but the HD5770 isn't too much more. It's well within your budget and if you are looking for something to last you as long as possible like you say it is the way to go.
for your current monitor, i would say stay with your card, it's still strong enough to play games with 1440x720 resolution...
Did you try all of your games at HIGH setting? How's the result?
I made a mistake on the the LCD display it is 1440 X 900. Not that this will make a big difference. I have not really had any real problems with my games at the high settings , but I have adjusted my settings downward on a couple of games, when performance was bad.
I was thinking about upgrading now, because game requirements always seem to be increasing and
I may end up keeping my system longer than two years. Since my motherboard has PCI-E 16X slot and
the newer boards are PCI-E 2.1 (which I think that I read is not backwards compatible), I thought I would a better selection now to upgrade for my future gaming.
So at the moment, I'm still debating, whether it will be cost effective to upgrade or not and not having to worry about future gaming.