A Radeon 6850/6870 is still more than sufficient for 720P, it will go toe to toe with a Radeon 7750/7770 and Radeon 250X, it's not going to last you "years". - But it should do fine for another 12 months of so.
What is truly holding you back is that Core 2 Duo and 4Gb of Ram.
I suggest you hold onto that Graphics Card for now and put your focus towards the CPU, Motherboard and Ram.
You *could* find a high-clocked Core 2 Quad second hand for cheap (If your motherboard supports it, I would need it's model number to know for sure.)
And drop in another 4Gb of Ram if you have the slots. (Or upgrade your current sticks to 4Gb sticks.)
Or, look towards an AMD CPU/Motherboard combination on the cheap, it's better price/performance in the low-end, unless you intend to overclock where Intel's Pentium Anniversary can be a blatant steal, even though it's "just" a Dual-Core.
If you *were* to upgrade your Graphics card and leave the rest of the system as-is, you really won't be changing your current gaming situation much, you will get higher framerates in games that aren't CPU heavy (Such as Diablo.) allowing you to turn on more bells and whistles such as AA, but in games which are CPU heavy? They would still be unplayable.
Games are also becoming increasingly more threaded, making a quad-core a minimum for any serious gaming machine, games such as Civilization hammer this point home.
--::{PC Gaming Master Race}::--
