I was in a similar boat!
I game at the same res and had a 280 which i recently chopped in for a 470, when i got the 280 2 years ago some said it was overkill, but to be honest when it comes to PC games i dont think there is such a thing, and a top line card will always be tested by some games even at lower resolutions
I liked the 280 and thought it was great, it ran games at high settings with smooth and generally stable framerates, but like others have said there are games out there that will tax any card on high settings even at more moderate resolutions, back in the 280's day Crysis stands out as a prime example, but there were others too
Now i got my 470 up and running i expect it will do a good job for another 18 to 24 months like the 280, but i'm in no doubt that there will be a fair few games in that time period that will push it even at 1680 x 1050
I had a E8500 with my 280 clocked in at 3.8, i recently upgraded to a core 2 quad 9650 (same time as i did the card, 2 days ago in fact) and have cloaked that to 3.8 aswell, like you i pondered the point of spending big bucks for a old teck CPU since the CPU was similar in price to modern I7 CPU's!
I went with old teck!, reason being is after some quick research sandybridge will likely be upon us and after taking a few months to bed in, will likely be the mainstream option in 12 to 24 months time, so going for a full system refresh new mobo/memory etc seems like not a good idea right now unless the old system could not be upgraded to handle another 18 to 24 months of top wack gaming, as it's better to buy in new mobo/mem etc at the start of a new CPU architecture rather than near the end which is were the I3/5/7 seems to be now
I wish i had spent a little extra and got a quad 2 years ago to be honest as then i'd just be looking at a card upgrade now!
After a bit of browsing i came to the conclusion that a quad OC to 3.4 or better would be more than enough for the next batch of games and a card upgrade from the 280 would see the graphics side handled aswell
I would not get a new card unless you do the CPU aswell, a dual core will hold you back, my dual core was great when i first got it, but in the last 6 months i have noticed a steady increase in the number of proper quad core games floating around the shelves as in, games that pay more than lip service to a quad core and actually run noticably better on one, Bad Company 2 and Dragon Ages are two examples of games i have recently got were you get noticably better performance from a quad than you do from a dual, even a highly clocked one
I would recomend you get a Q9400, overlcock to 3.2/3.4 which should be easily do= able upgrade your PSU to 600/650 as my 470 needs a 550 W suppy as a minimum according to the requirements box and get a 460, that should see you in good stead for the next 18 odd months gaming wise